I get a lot of arrangements from Choral books and just adapt them to fit.
Most arrangements however are just songs that I feel like go well together.
You start with a theme – say The Cross and pick out songs that fit that theme.
Next you look at key relationship – you shouldn’t jump around in the key signature – use related keys like Ab – Eb or G – C. Obviously the best way is to use songs that are in the same key.
For instance the last worship set we did on Wednesday Night was entirely in Bb – so there were no needed transitions etc.
It’s good to go up a half step or a whole step – just make sure your key instrument can make the transition without a hiccup.
The last thing to consider is TEMPO – if you change tempo drastically there should be either a clean break or a verbal transition – so the musicians can change tempos without it being distracting.
One other thing is don’t overdo it or force something that isn’t there – It’s better to just stop a song and start a new song than to try to force it to work.