There are more puggable raids in WoW than there have ever been. Everything was puggable before 3.1 came around, and with a decent group there are still puggable bosses in at least Ulduar 10 (25 may be a different matter).
If you are leading these PuG runs, you will want to institute a fair loot system that is going to be easy to keep track of. This is a PuG, and trying to use systems such as DKP are just going to make it so that people are not going to run with you. DKP is for guild runs, not PuGs.
There are many different methods you can use, and none of them are right or wrong. It is just a matter of preference.
Method 1: One need, One Tier, Rest Greed: This seems to be one of the most common methods in any pug raid you will go to. The raid leader will set it to master looter. You can win one "need" roll until the people you share gear with also win a "need" roll. After that, you will have hope that it falls into the greed category.
Tier Items works the same way. You can win one until everyone else has won one. Then it starts over.
Overall, this seems like a very fair method. The only thing that may cause discontent is that people can attempt to abuse the system by passing and taking their chances on a greed roll.
Method 2: One Need, One Tier, One Greed: This method is more or less the same as the above method with one big change. It makes it more difficult for people to save their need rolls as they only get one greed as well. If this were a guild method, the problem would be that people would pass on upgrades because they would rather pick up another upgrade, therefore making the raid weaker. In a pug, this can just mean extra nexus crystals for your bank. If you are the leader, keep the shards. People will be more hesitant to see it not used.
Method 3: No master looter. Need or Pass: This method is not nearly common, and is more likely to be seen in a Naxx 10 type PuG were everyone already overgears the instance. The premise is pretty simple. If you can use it for either of you two main spec, need it. If not, pass. The Raid leader will shard whatever is not taken. It is up to you to decide if you want to pass out crystals or just keep them. Whatever you decide, make sure you make it clear before you start the run.
Method 4: Free Roll, Use discretion and do not be greedy: All loot for your dual specs are up for grabs. As the Raid Leader, you will have to use discretion on who gets items. This method has fail written all over it and is very hard to enforce. It could look like favortism if something looks unfair. You are probably better off going with a standard Loot Council type system if you are looking to do this.
Method 5, Free Roll, No limits: If you are using this system, it is basically up to the loot gods and the RNG. Make sure that you keep it on Master Looter to keep players from needing every single item in the instance. Just because it is free roll, is not an excuse for people to be retarded (aka a warrior tanking caster gear).
Method 6, Join A Guild: Not really a Loot Method, but certainly saves you from the hassle of having to choose from a different loot system and herding the cats through an instance :)
I will be coming out with more rules for Leading Pugs soon.