If you want to kick up all your solos up a notch, place it last. This way you're boosting your modified tone.
If you want to kick up your distortion/overdrive pedals, place it first.
Of course that's the basic guidance, but it'll all depend on what board you have, what you like to do with it, which pedals like a boost in front etc...
I wish these boosters had a buffer built-in... I'd place one first and one last in the chain!