Whenever designing a DKP system, I think it ultimately helps that it focuses around to create high morale within your members rather than putting limitations in order to min/max your raid. Having people that will show up every night and work hard at making progress because they are happy is more important than making sure every DPS item goes to DPS first. Of course, you can't have all of your non-set leather going to your druids if your rogues don't have any upgrades yet.
It will be up to you how you want to do it, but there are a few ways to create restrictions while creating some morale in your ranks.
1. You can put restrictions for all of the items that drops in the instance, but once the instance is down on farm, the restrictions are lifted and the items are open bid on items.
2. Your list of restrictions stipulates that the item must drop 3-5 times before it can be bid on by the other classes.
3. You have open bids on all items. The only time you should do this is when you have people that are selfless and focused towards not only progress, but making sure everyone in the raid is also happy.
Personally, I'm a fan of open bids. DPS classes may get pissy when healers want to bid on something they will not use inside of a raid, but in most cases their desire to block healers from getting DPS gear is more based on personal greed over raid progress. In most cases, it isn't hard to find DPS for your raids, but it's always a struggle to find healing due to the healer burnout that occurs quite often. And if you don't know what healer burnout is, it's when your healing classes, especially hybrids, are forced to do nothing but heal and are only allowed to bid on healing items. Eventually it makes the game boring because they can't speed up their rate of farming or go WTFPWN people in BGs or world PVP. Add on top the stress that comes with the job of healing in raids and the game just doesn't become enjoyable anymore.
That's why you need to create your DKP system to create a sense of hope for these people so they know they have a chance to get the items they actually want. You want them to gear for raid healing, but you want them to know that if they have put forth a lot of effort into progress that they deserve to have the fun items. Because if they aren't having fun, they'll quit and then you won't have a raid unless you can find more healers which isn't as easy as you'd think.
However you decide to put restrictions on loot, you need to make sure people aren't bidding on items their class can't use at all because in most cases they are going for a nexus. In otherwords, no bids on downgrades, Warriors take warrior shields and pally/shamans take shields with int. Stuff like that. But in doing restrictions, do some research and get a general scope of how classes function. For example, you might have some casters that get pissed at a pally that bids on a +damage +crit sword, but it's very logical for a pally to want it for his/her healing due to the fact that they have a talent that lets them heal mana free when they crit. And right now, there is only one healing mace that actually has crit on it at the moment if I remember.
Now for an actual way to run the system in regards to charging points for items, I'm a fan of silent bid. Make a bid what you feel the item is worth to you and go from there. Reason for that is it gives people a feeling they have a chance to win items. Though I find the best way to dampen inflation is to charge the winner the amount of DKP that the second person bid. Really the best way to figure out what works best for you is to look at the loot systems of a lot of other guilds, mostly the more successful long running ones, and sort of base yours off of it.
I think I've went on quite a bit and probably too much than a should have, but I will finish by saying to remind you that you will make constant changes to your DKP system as you go based on the needs or problems that occur in your guild. It may be changes in DKP awarded in order to encourage attendence or it may be related to some other issue.