When you receive your invite, check your calendar, and if necessary, inquire with your work about time off, check into travel arrangements, etc. You should RSVP with immediacy. Do not let it sit in a pile with your other mail. Important decisions, some of which have financial consequences, are resting on the wedding's guest count.
No matter what you RSVP, inquire with a family member of close friend of the bride and groom where the couple is registered. As much as possible, it is always nice to try and buy a gift off of their registry. Often, these are the things couples need to start their home. Some people wonder how much they should spend, and while price is not important, you should, if possible, buy a gift that is somewhat proportional to your relationship. Haven't spoken to the bride since college? No need to buy the espresso machine. Groom's uncle? Yes, you should probably go bigger than just the salt shaker. If you can, mail the gift rather than bring it to the wedding. It's perfectly fine to bring it to the wedding, but it is usually easier for the couple if it arrives at their home.
Even if you think it'll be fine, do not invite anyone extra who was not listed on the invitation, or bring an additional guest when you passed on the plus one invitation you may have been offered when you RSVP'ed.