For 25 man raiding, we have a fairly straighforward DKP system.  We use an addon, QuickDKP, to help manage it.  The addon uses officer notes (viewable by Raiders) to keep track of everyone's DKP.  Typically one officer will run the addon and another will handle master looting for the raid.  DKP is earned by the hour; each hour that you are in the raid, you accumulate 5 DKP.

How to spend it!

• All items that drop from a boss will be linked initially in raid chat.  We do this because not everyone wants to bother looting the corpse, and this way everyone can see what dropped.
• Each item will be linked individually in raid warning chat.  When you see the item flash across your screen, if you want it, then it's time to bid.
• All bids are silent.  Whisper the master looter your bid.  The highest bid wins for the second highest bid +1.
• If 2 people bid the same, the person with the most DKP will win the item for the amount bid.
• If 2 people bid the same, and they have the same DKP, the 2 people will be asked to roll off.  Highest roll wins.
• The minimum bid is 5; there is no maximum bid.
• If you have less than 5 DKP (including negative DKP), then you can always whisper a bid of "min."  "Min" as a bid is special; it is used when you kind of want the item, but don't want to take it over anyone else.  If you have less than 5 DKP, you can use "min" as a bid because we'd rather see you go negative DKP than have a useable item rot.  The point value of "min" is 5, but it is not the same as a bid of 5.  If the master looter receives 2 whispers, one "min" and the other "5", the person who whispered 5 will win the item.
• Offspec bids are not regulated strictly.  We do ask that mains get first dibs on loot, and we will not condone something like a tank bidding astronomically for a prime DPS weapon over DPSers.  That would ill-benefit the raid.  If you would like an item for your offspec, as a rule of thumb keep the bid low so that we (Officers) don't have to intervene.

Here's an example of a possible loot scenario, to help hammer the DKP system home.

Raider1 bids 34, Raider2 bids 5, Raider3 bids 75, and Raider4 bids 52.  Two of the same item dropped from a boss (like 2 tier tokens from Maliorak).  The top two bids will win the legs for the 3rd highest bid plus one.  So Raider3 and Raider4 will both win the item for 35 DKP each.