I usually decide my playthroughs by going through the following checklist:
1. Male or Female?
I try to play 50/50 split between a male and female character. This also depends on who I want Shepard to romance (if any) during the game. It my last few playthroughs I have been playing Shepards that romance characters I had seen play out before (such as Tali or Steve Cortez), so this makes choosing a specific gender easier.
2. Class.
A lot of this depends on what I want to play in ME2 or ME3, since I have more fun and playstyle choices in those games than I do in ME1. I like to mix things up and try unique builds or playstyles to see what works. I will usually test it out first with an old save beforehand before I commit to a whole playthrough with it.
For example, I plan to play as a Shepard in ME3 that plays like the Female Quarian Engineer in ME3 MP (Sentry turret, Cryo Blast, Incinerate), so I will be playing as a female Engineer.
3. Suicide mission.
Right now I am doing some "themed" suicide mission outcomes, like pro-biotic, anti-biotic, pro-tech anti-tech. I know players who have done pro-human, pro-alien playthroughs, or 'only Cerberus', or all-male, or all-female outcomes.
So I look at this thread before starting and try to map out how I want to achieve the desired outcome. (Also a good idea: make sure to save before starting the suicide mission or even the Reaper IFF mission in case it does not work out as planned).