Three techniques:
1. Move your open hand towards the fly very slowly until you are 5cm (2 in) away then slap.
2. Move both hands slowly towards the fly and "clap" to catch it.
3. Blow gently downwards onto the fly, then slowly increase the force of the blowing. You might then be able to pin its wing to the surface.
Yes, I have a lot of time on my hands.