MOST IMPORTANT RULE OF INSANE IMPORTANCE THAT THOU SHALT NOT BREAK:
Enjoy what you're doing.
Whether you're playing through a game you love or ripping a terrible game to shreds, you MUST enjoy recording it. This is why blind LPs are risky: You don't know if you'll enjoy it.
Less Important (but still important):
-Decide now whether you want to do a text/picture LP or a video. Text/Picture works well with slower paced strategy games, video is required for FPS/Action/Horror games, and everything else can go either way (Myst or Elder Scrolls, for instance, can be done either way). Make sure you play to the strengths of the format. For instance, a text/pic LP of Uplink should focus on the strategic approach you take to the game, while a video LP of Uplink should focus on the pulse-pounding hacking jobs themselves.
-Decide on vocal quality (if you're doing video). If you hate your voice, stick with atmospheric games that don't require talking (Malorie's LPs of Alone in the Dark and The 7th Guest, for instance, use text overlay on the video). If you can do funny voices, feel free to use them, but be aware that many people will hate you and others will love you for it (see PewDiePie). Otherwise, stick with a generally professional tone and don't drone.
-Learn when to shut up. Nothing wrecks a climactic battle like someone yammering about boobs in the background.
-Start short. This is where I royally screwed up: I tried to do an LP of Daggerfall, and while I put out an admirable amount of content, I didn't even get a third of the way through before it became too arduous. Had I played something like Tomb Raider instead, I would have finished it easily.
-Don't be afraid to cut the crap. If you're struggling to do one section over and over, if you're doing busywork to get to the next part of the story, or if you're wandering around dazed and confused, it's time to grab the pruning shears and get editing.