He speaks the truth!Toxic Sniper said:Have a plan for what you want to do with the Let's Play beforehand. Blind Let's Plays tend to be bad UNLESS you can grab somebody who has already played the game to help you with it.
Speaking of which, Let's Plays become 40% funnier whenever there is a second person voicing over. Dialogue tends to bring out jokes, and the second person will often see things that you missed.
Try scripting some jokes beforehand or getting some practice speaking. That can get rid of your "um" and "ah".
Preparation and polish are everything. The Let's Play market is heavily oversaturated, the chances of you getting any success in it are about the same as winning the lottery, so you must do absolutely everything as perfectly as possible: Videocapture + sound quality must be the highest you can get it, the gameplay itself must be as entertaining as you can make it, your voice must be as clear as possible, no mumbling, umming, very long silences or sound interference (dogs barking, family interruptions trains passing by the windows etc.) Preparing a script, or at least a few jokes or interesting things to say will help the flow of the video immensely.
Make sure the video starts and ends well; don't use a 20 second long MS Paint intro slide with grating guitar music, if you must use an intro, make it as short and intrusive as possible.
Finally run the final video past a few friends, if they aren't enthusiastic about it then you probably shouldn't bother.