Tippy said:
Acronyms and pronounceable 4 letter domains are a safe bet.
It is a safe bet that all but the most obscure acronyms and pronounceable are already taken.
My favorite saying regarding LLLL.com is that a random LLLL is an acronym that I haven't yet found. That said, the idea is to maximize the chance that the name(s) you get will be "found" - that they will be most likely to be the initials of a company that is willing to spend money for a domain.
The least often used letters are Q,X,Y, and Z. While names with those letters do sell, the odds increase without them.
The next tier is names containing one or more W,J,K,U or V. Any name you find for registration will contain at least one letter from these two groups. Interestingly, this has been true for at least a year and a half - an indication of the power of the superstition that there is a hard line between "premium" and "bad" letters.
Anyway, I would recommend you look for names with one (or rarely two) letters from the second tier and where the other letters are as commonly used as possible. One possibility would be to type the names you are considering into Google to see possible acronyms and the total number of hits. However, most of the higher hit count names are long gone.
LLLL.com names have been carefully searched through by many people over a long time. Coming into the game now you do not have the selection previously available. However, you do have the advantage that you did not have to pay renewal fees on your names for all that time.
It is expected that there will be a lot of interest in LLLL.com names as the last ones are taken - probably some time next year. They offer a small risk and the posibility of a good reward. Check out DYYO.COM for more information and a great search tool.