Some posts are just not going to get tight without stripping themselves. I had this problem and converted my posts to WBT's. Now no problems. A couple hundred dollars later of course (darn BiWire!)
I use an appropriately sized socket (without the wrench) to hand tighten.
In addition, where the speaker cables exit the spade, I will point the cables at about the 4 o'clock position (instead of straight down at the 6 o'clock position). As I hand tighten using the socket, I'll using the other hand to turn the spades/cables toward the 6 o'clock position simultaneously.
This method seems to work quite well at providing a very snug connection between spade and binding post.
Stehno's suggestion is the same method that I've used quite successfully for many years. Try to get a grip with a socket around the post (just use your fingers around the socket - don't torque too liberally with a ratchet driver or you may strip the threads).
Also, place the spade lug onto the post at about 45 degrees counterclockwise from the desired final position. As the post rotates tighter, turn the spade in a clockwise direction along with the post. They'll end up at a much tighter final position by doing so.
X-terminators add an interface level and, while convenient, slightly veil the sound.
The AQ binding post tool is the goods - highly recommended.
If you cannot tighten them even with a socket or the AQ tool, something may be broken as Ncarv points out, or you may have to get inside the speaker to tighten the nut on the crossover side of the post - had to do that on my DSigs when I first got them.