Let's take a look at what cards a typical UR Delver deck contains. In order to reliably flip a Delver of Secrets we need to run a high amount of Instant and Sorcery spells, around 25-30. We also want a low land count (18-20), which means we can only run a few creatures and must keep the general mana curve low.
Creatures
Delver of Secrets - The creature the deck is built around. Ideally, we want to play a Delver of Secrets on the first turn and flip it on our second turn, creating a 3/2 flying creature that must be dealt with. Amount needed: 4 is a must.
Snapcaster Mage - The deck will have lots of spells going to the graveyard, and being able to flashback them as needed is really important. One of the best blue creatures ever printed. Amount needed: 3-4, leaning towards 4.
Young Pyromancer - Since we have lots of spells, and cheap ones, it's easy to create a decent token army to either use as a chump blockers or as an attack force. Amount needed: 3-4, leaning towards 4.
Grim Lavamancer - One of red's best one-drops. Gives you repeatable removal, and a way to get more use out of your graveyard. But it isn't a vital creature like the first three. Amount needed: 0-2.
Vendilion Clique - An aggressively costed 3/1 flier that can be flashed in, and that can either filter one of your cards or get rid one of your opponent's cards. Not essential, but good to have. Amount needed: 0-3.
Spells
Gitaxian Probe - A "free" cantrip, that also gives you information on what your opponent is holding. With Young Pyromancer on the battlefield you can create tokens while keeping lands open for important counterspells or removal. Amount needed: 4, very important.
Serum Visions - Another cheap cantrip, that can set up your future turns as well, and thus help flip Delver. Amount needed: 4, very important.
Lightning Bolt - Needs no introduction. Amount needed: 4, no reason to use fewer.
Pillar of Flame - The exile clause helps get rid of creatures permanently. Amount needed: 2-4, you probably want a few somewhere in your 75.
Magma Spray - Same as Pillar of Flame, but this is an Instant, which might be relevant sometimes. On the other hand, it only hits creatures. Amount needed: 0-4, use if you want the instant speed and isn't bothered by not being able to burn the opponent.
Remand - Counterspell and cantrip in one. Of course, it isn't permanent, but a one turn delay might be all you need. A great tempo spell. Amount needed: You probably want the full 4.
Spell Pierce - A cheap counterspell for those early turns. Doesn't target creatures, and becomes less relevant the longer the game goes. Amount needed: 2-4, you want a few maindeck, but probably not the full set.
Spell Snare - Another cheap counterspell, but very limited. However, there's several spells in Modern with a casting cost of 2 that you want to get rid of, for example Tarmogoyf and Bitterblossom. Amount needed: 0-4, a bit meta dependent.
Mana Leak - A hard counter early, but less relevant in the long run. Amount needed: 3-4.
Vapor Snag - Unsummon variant, but due to the damage clause it's preferable to Unsummon. Bouncing opponent's blockers and saving your own creatures (or reuse Snapcaster Mage) is important. Amount needed: 3-4
The rest are more optional, but you might want to consider some amounts of them somewhere in you 75
Magma Jet - A bit expensive for the damage amount, but helps you set up future turns and thus flip Delvers.
Forked Bolt - A 2-for-1 removal spell, might come in handy against spirit tokens (Lingering Souls) and faeries (Bitterblossom).
Arc Trail - Same as Forked Bolt, but a bit more expensive. Takes down a toughness 2 creature though.
Electrolyze - Another 2-for-1. This time an Instant and therefore more flexible, also a cantrip. But more costly at 3.
Izzet Charm - Flexible, both removal and counterspell. Third mode might not be relevant in this deck.
Negate - A hard counter against non-creatures. Perhaps not maindeckable.
Essence Scatter - Negate's companion spell, against creatures. Perhaps even less maindeckable.
Burst Lightning - Another cheap burn spell, that becomes better in the long run.
Land
Steam Vents - You need both colors early, so 3-4 of these are a must.
Scalding Tarn/Misty Rainforest - Blue fetchlands are preferable, because Islands are more important. Use the full set of 8 if you have them.
Sulfur Falls - Another dual land, but it's probably not worth running the full 4 set of this, because you often don't want to start with a tapped land on your first turn.
Mutavault - Gives another threat, but 2 are probably maximum since you don't want it on your first turn.
Faerie Conclave - Another man land. Gives a flier, but comes tapped in play.
Ghitu Encampment - Worst of the three man lands worth considering. But I think the first two choices are always going to be preferred.
Island/Mountain - Basic lands are needed, and you want to run more Islands than Mountains.
Artifacts
[card]Runechanter's Pike[/card] - Since we run a lot of spells, this equipment can become a real force. Cheap to play and equip, and the First Strike is very relevant. Use 1-2 at most.
Sideboard options
Hibernation - Can set back green based decks, like Gruul Zoo, and a way to get rid of Hexproof/Shroud creatures that are common in green.
Flashfreeze - Modern have several often played red and green spells. If you face lots of decks based around red and green, Flashfreeze is a sideboard option to consider,
Dispel - Cheap hard counterspell, but also very limited. Use against control decks featuring lots of instant spells.
Counterflux - When you really want to win counterspell wars.
Shattering Blow - Affinity decks always have to be considered in Modern, as well as Spellskite. So anti-artifact spells have to be considered. Shattering Blow exile the artifact, which is best against Wurmcoil Engine since no tokens gets left behind.
Vandalblast - A cheap spell early, and a blowout later. The Overload cost at five is a bit high for this deck though.
Smash to Smithereens - If you want to deal some damage at the same time.
Shattering Spree - Can destroy several artifacts at once, but since this deck leans towards Blue it won't be so effective. However, a 2-for-1 or 3-for-1 can be enough.
Blood Moon - The deck has many basic lands, which means Blood Moon might be more crippling for the opponent.
Combust - 5 uncounterable, unpreventable damage for 2 mana is nothing to sneeze at. Will be even more important against Faeries.
Threads of Disloyalty - It's limited and dies to the commonly played Abrupt Decay. But stealing a Tarmogoyf is always nice.
Flame Slash - Cheap and gets rid of bigger creatures than Lightning Bolt. But Sorcery speed and only being able to hit creatures makes it sideboard material.
Sample decks
Carlos Moral's Top 4 deck from GP Prague 2014:
[deck]Land
6 Island
4 Misty Rainforest
1 Mountain
2 Mutavault
4 Scalding Tarn
3 Steam Vents
Creatures
4 Delver of Secrets
1 Grim Lavamancer
4 Snapcaster Mage
2 Vendilion Clique
4 Young Pyromancer
Spells
4 Gitaxian Probe
4 Lightning Bolt
2 Pillar of Flame
4 Remand
4 Serum Visions
2 Spell Pierce
2 Spell Snare
3 Vapor Snag
Sideboard
2 Blood Moon
1 Grim Lavamancer
1 Hibernation
2 Magma Spray
2 Negate
2 Shatterstorm
1 Spell Pierce
3 Threads of Disloyalty
1 Vendilion Clique[/deck]
Gameplay videos
GP Prague 2014 Quarter Final:
http://www.youtube.com/watch?v=h-Phgd5gmWU
GP Prague 2014 Semi Final:
http://www.youtube.com/watch?v=a7tPeWgesqU