One method I like is to give the name and type (animal, outsider, humanoid, etc.) with the baseline successful check, along with all useful information about that type. Granted, just knowing what the type is is not always helpful, but it can be useful in distinguishing between a native aberration, say, and an outsider that can be banished. I agree with DM_Blake that giving ALL subtypes and information about them on just the baseline roll is way too much. If the type and name tells them absolutely nothing useful at all, I might throw another tidbit in there.
For each additional 5 over the DC they can ask questions. Valid questions are pretty broad for me, though. Here are some examples:
* What are its subtypes, and the properties from subtypes? [If they ask this, I will give them ALL subtypes, and select ONE subtype that is most useful for them to know the details of, such as 'devil' for example, rather than 'evil'. For that subtype they will learn the properties shared by all devils. In almost all cases there is only one "useful" subtype.]
* What special attacks does it have? [If they select this, I will tell them all special attacks if it has only a few, or a selection of the most significant ones if it has a lot.]
* What spells or SLAs does it have? [As above]
* What are its defenses like against damage? [This will give them elemental resistances, as well a general sense of its armor class breakdown]
* What are its defenses like against magical effects? [This will give them a sense of its saves, and immunities to specific types of magic]
* What is its most dangerous quality? [Because this is such a targeted and useful piece of information, I only give them one thing, but I select it to be the one thing that is most threatening to them]
* How does it move? [This will get them all move speeds, as well as any teleportation or movement abilities]
Other questions are possible as well, and note that some of the above overlap in their information. For example, asking about the devil subtype and its defenses against damage will both give you some information about elemental resistances.
Further, and this is my favorite, if they beat the DC by a full 20, which is not easy, but I've seen it done, then they get to just flat-out read the monster's entry. They know everything about it. Players really like this, and I like it as a special reward for someone who really invests in knowledge (and you have to to ever beat the DC by 20).
Roleplaying wise, I like to ask my players how they know something after I give them the info. So, the Fighter [Lore Warden] might learn something about a troll, and I'll ask how he knows. A good roleplayer might say: "Oh, right; I remember hearing about that from one of my sergeants back at the academy. He lost half a hand to one.", for example.
I should also add that I assume players carry-over information shared between monsters. Like, once you show that you know the properties of devils, then you forever-after remember the properties of devils. So, if you encounter another devil, as long as you can determine it *is* a devil, then you automatically know devil properties for it.