Scout is all about learning to adjust to his mad speed, and being able to still aim competently. This is a hard learning curve, one of the hardest in the game IMO, and because of that and the scout's tendency to...die alot...he is one of the least played classes.
Chupathingy is right of course, there's no great secret tips to impart, because it's all about practice. You'll start to learn to use the scout's speed to
help your aim instead of hurt it. Remember: with scout, his elusiveness is as important a weapon as his scattergun. Use that speed to slip around behind your enemies and get a good scattergun blast in their back from point blank. After that all you need to do is run around at mid range and cover them with scattergun (watch those 6 rounds!) or the pistol. Don't forget the pistol! It is your friend. Or if you're me, you probably just pull out your bat and try to finish the job in style, for better or worse

Of course everything changes by class your facing. Notice how your enemies kill you, and remember that for the next time. Demo and Soldier is all about running in unrpredictable patterns. If all you're doing is strafing left and right, a good player can simply lob a grenade or shoot a rocket into your face as you run. Never run straight at them. If the demo starts getting sticky-happy, back off quick. The rocket and grenade launcher only have 4 shots, and they will use them up quick. Exploit that reload like there's no tomorrow. And watch the bottle/shovel.
Run from pyros. Learn their flamethrower's max range. It's bigger than you think it is.
And remember you're purpose! Odds are, you're not going to rack up the big points as Scout. So make sure you know you're doing your share by capping points, relaying information to your team (you are a SCOUT after all), and taking out key enemies (like MEDICS!)
Hope this gives you a decent starting point, I highly advise checking out this video. EvilDaedalus is some kind of superhero.
http://youtube.com/watch?v=jRSW0Y1SgoA