"Thank you. I have other tentative plans for that evening, but if those plans fall through I'd love to come! Will the offer still be open if I'm able to come last minute?"

Thank you for thinking of me. I will have to check my calendar and get back to you. Meanwhile, what are you thinking about ... (change subject to ANYTHING)

It seems like you've been invited because you've professed an interest. That's a tough one. There are several levels of response that you could give...list them out and then pick the one that best suits the situation.

"I'd love to but am looking for something better to do...like maybe sit at home reading a book." Ouch.

If this is a formal business engagement, does it involve a lot of planning and monetary output on the part of the host? Are they interested in selling you on something? What is it that makes you hesitate? Will you be expected to talk positively about their views and/or business while there?

Basically, I think that simply explaining that you aren't sure you can make it then inquire if they need a definite answer and by when. Remember to thank them for the invitation if you want to keep your options open. If you are leaning towards going but don't want to make a definite decision, express an interest in any other future events they might be hosting while telling them you're not sure whether you can make it this them. Do not add the latter if you are leaning towards not going and your interest in the subject/people has been waning. Again, if you are leaning towards going, having them expend more energy on getting you to give a definitive answer is less rude then if you are leaning towards not going...where their energy and time is likely being wasted when they worry about whether or not you'll be there.

The line that has saved my butt on many occasions:
"That sounds great, but I will have to check my schedule and get back to you."
I never immediately respond to any invites anymore.