There are two ways to deal with this problem. The first is to totally forget about it, and it will come to you later. Of course, you don't want it later, do you? You want to remember it now. So you go to the second method.> You think the alphabet through in your mind. It's best if you do this in a sort of dream-like state. In other words, don't get hung up on what you're trying to accomplish: Just think through the alphabet in a little sing-song way. You will hit upon a letter that makes you stop and think, "Hmmmm."
If this letter doesn't trigger the word you're looking for within a few seconds, it is probably wrong. However, it is almost certain that this letter is in the word you're looking for. So file this letter away and continue through the alphabet. If you get all the way through the alphabet and cannot find another letter that seems to fit, then go back to the only letter that seemed to fit and start working with syllables. For instance, if the letter you kept was S, try sa... sc... se... sh... etc. with any syllables which could logically begin with s.
These sorts of mental exercises are fun, if you don't let them aggravate you too much.