I seem to be doing a lot of callbacks to old posts lately, but new information does come up, and I’m always willing to re-examine old data, in the light of new discoveries, to see if a thing still works, or needs to be turned over and rebuilt.
This is something that a GM should be willing to do, and do often. Keep things interesting, keep things, as they say, fresh and especially, keep them balanced. This is not, and should not be treated as, a reason to throw out things that work. If adding new rules only makes things weird and confusing, don’t do it – remember, the point is fun, not frustration – unless it really is necessary for some reason. If that is the case, make the transition as simple as possible. This also applies to dumping rules – while I am a great supporter of ‘this is dumb and clunky, dump it’, there are times when that isn’t the right way to go.
The most likely source of new rules is going to be a player; it’s not at all unusual to hear a variation on ‘I found this in this book, and I really want to try it!’ – where ‘this’ can be anything from a race to a class or feat, and ‘this book’ can be anything from a source book you aren’t using, or a third-party book you’ve never heard of. Some game companies have a butt-ton (an actual measurement!) of material, some less, and then there is third-party material that may or may not actually be useful. Read everything you’re presented. If you can get your hands on the actual book it came from, read that, too. Be ready to say ‘no’ a lot. And even if you allow it, be aware that you should reserve the right to change your mind if whatever it is proves to be wildly game-breaking.
We’ve generally found Kobold Press and Dreamscarred to be good sources for third party materials, but even those we use sparingly, if at all. Your mileage, of course, may vary.
House rules are the other category of things that must be revisited often. House rules can get unwieldy very quickly, that appears to be true regardless of system, and sometimes, you really do need them. I know I have a short list of house rules that get applied to most of the games I run. If I can sell a GM on them, they tend to get applied in games I am playing in too (I have found that a given character’s utility is vastly increased with just a small adjustment in their skill point loadout…), but I have also seen house rules that made certain characters /too/ powerful. So… revisit things. Be willing to adjust. Sometimes a thing is not going to work in play. And if a player tells you something isn’t going to work, it isn’t a bad idea to hear them out; might save you a headache down the line…