ah the tip of the tongue phenominon... there is something that almost always helps me out when that happens, it's a hypnotical strategy..
Stare at something steady, but will calm you, like a flame...a clock on the wall...get very comfortable, and completely relaxed...seriously it really works
you can keep your eyes open or close them if it helps, and imagine a blackboard...do this for a long time, and imagine that what you wanna remember will be written on this board... it takes a good 15-30 mins for this to work though, so be patient if it doesnt come right away... try to clear your mind but keep imagining the black board...
[edit] this also helps when you are taking a test and you have time on your side...you can remember formulas, names, all kinds of stuff..but it takes practice