Most of whats been said already ... really REALLY hot water with good car shampoo (autoglym do a bug removing one) and use a sponge enclosed in mesh and as SJRM has said ... soak em first! Once you've got them off get some decent wax on it ... the little sods come off easier if the car's waxed.
You used to be able to get a small sponge called "Bug Shifter" which had a non-scratching plastic scourer on one side which works well. If you cant find one of them any kitchen or bathroom Non-scratch scourer sponge should be safe to use as long as you do plenty of soaking first and then a good Autoglym afterwards.
Plan B ........... Respray the car in "Fly Snot Green" !!!!!!! :read2:
I used a spray bottle with water in today, sprayed the water on and left for a few minutes, sprayed some more on then used one of those sponges with the nylon netting fixed to it - dead flies gone in seconds. :wink: