I have a fitness/ropes course in a tree on my property. If I post a "at your own risk" type sign, am I protected from potential injured trespassers?

You don't say whether your ropes equipment is for your personal use or if you are allowing/encouraging others to use it.  If the former, you should set up "no trespassing" signs and/or "keep off the property" signs. If, however, you are inviting others to use the ropes (whether as a paying customer or as a means of entertainment), the situation is very different.  In that case you WANT people to use the ropes and you are asking is there a way to limit your liability if someone gets hurt.  There is no way to absolutely protect yourself from a lawsuit if someone gets hurt on your ropes.  The only way to even limit your liability would be to require each user to sign a release before using them.  The document should be drafted by a competent attorney, but even then, there are still possibilities for suits - even successful suits.