Bachelor and Bachelorette parties vary greatly in many ways... from the level of formality, to the number and type of guests, the activities, and of course, the level of raunchiness. The most important thing to remember is that this is a special night for the bride or groom, a final send off from single life, and those who host the party should be considerate of what the bride or groom would like or dislike.
If the they would be extremely embarrassed by a stripper, do not hire a stripper. Sounds pretty logical, yes, but some people have the misconception that the purpose of a bachelor or bachelorette party IS to embarrass the bride or groom. This should not be the goal. A little light humor is one thing, but this is an event that they will long look back on, so be courteous in that respect.
I love bachelor and bachelorette parties because, to me, they are a deeper event than just a deboucherous night of pub crawling. That may be involved, but for the ones I have thrown I felt an obligation to make at least slightly more meaningful. In a sense, it is a good-bye. Friendships can easily remain after people get married, but they shift. The shift is meant to happen, because something should shift when you make such a huge commitment and life change. So in a sense this event is like a rite of passage. More detailed ideas that I have used (and have been a success with guests) are in bachelor and bachelorette specific posts.
A few general ideas for both:
* Don't feel limited to just the evening for the event. If you have the budget and/or creativity, it can be an all day celebration, or even a weekend long getaway.
* Ask the guest of honor specifics about what they want and do not want, and respect those preferences. Be particular... Who exactly do they want to attend? Do they want their mom (bride) or dad (groom) there? Do they want everything to be a surprise? What have they envisioned for this night?
* Do not throw the party the night before the wedding, for obvious reasons.
* Refer to the "general party planning tips" post, also.
* Remember to have fun, and keep the event as low stress for the bride or groom as possible. This is already an extremely stressful time for them. Allow it to be an event where they can relax and have fun.