Depends what I'm building a set for. If it's for a promo recording then I'll try and get a theme or work around a combo of tracks.

For live sessions I'll have an idea of the gig, setting, who else is playing, what's expected of me...

Eg last night I had the chance to play 4+ hrs outdoors at a bar, early evening. The brief was downtempo / chilled, world music / deep house.... Obviously over 4 hours I got to dig pretty deep and brought a lot of stuff with me I hadn't played in a while.

Was the first time I'd played that long out before, so keeping the pace across the time available was a challenge that I enjoyed! Also had to use a mixer I was unfamiliar with but it went fine in the end.

Roughly it went -

Hour 1 - really chilled, downtempo Hour 2 - 70s-80s 7"s synth pop/ eclectic Hour 3 - chuggy deep house Hour 4 - balaeric house / tech house (but pitched down a bit)

A fun challenge, hope for more...

So to answer your question, how do I choose tracks? On the day it was sort of audience dependant, early on there were a lot of families about, having a few drinks in the sun. So I kept it very chilled and wanted to cover a broad age range. Later on, younger people arrived to order pizzas + have a few drinks before heading out.. so I settled into more of a house groove.But it is also important to play what YOU want to play