The best advice I have is to get into the head of your opponent.

That means:

- Knowing what they're capable of:
A giant polearm spec isn't going to be able to surprise you like a giant sword spec would. However, you'd want a high wimpy when fighting a giant sword spec because it might let you survive a big flurry, whereas a high wimpy against a giant polearm spec would get you killed.
A STSF elf generally isn't great at dishing out or handling burst damage. If you can't do a lot of burst damage to a dexy STSF warrior, you'd best get away. Good news is, it's very unlikely they'd be able to permalag you or kill you unless you over-commit.
Druids and shamans and necromancers, etc. often want to drain you until you can't move or cast. Keep an eye on your moves as well as your HP as you fight.
Try to have an idea of what their strategy would be against you. What is your worst-case scenario? How would they make that happen? If you see them doing all of the things to make that nightmare scenario come to life, gtfo and be careful against them in the future because they're crafty!

- Knowing what you're capable of:
What would have to happen for you to win this fight? Do you need to make a sword spec drop a weapon to stop them from parrying so much? Are they tank-gods with noremove weapons that need to eat a bunch of burst damage outside of combat? Are they ABS-addicted mages who you need to lag to high heavens? As a general rule, you either need to nuke the melee abilities of a really strong melee character or lag and damage non-melee characters.

- Having a Plan B:
Before you start fighting, what is your escape plan? Can you outrun them? Is your escape potion in your inventory? Can they curse you? Do you have a teleport? If you have to run, could they have you trapped by quicksand or centurions or a snare or friends waiting outside?

- Picking Your Battles:
If everybody you fight is always prepped, that means that they're the ones who decided to fight you, not the other way around.

If you get attacked by uber-prepper warrior, run away, wait a bit, then try to rush them 12 hours later when they haven't had a chance to stack all of that damage reduction or mentally prepare for a fight. You can turn those tables!

In sum, you should:
> Know what your enemy can do to you, and probably wants to do to you
> Know what you can and should do to your enemy
> Have a pretty well-conceived escape plan in mind
> If they're running at you, it is probably because they think they're readier than you are. And if they are, run away and re-engage when it's even or you have the advantage.

Edit to add: There are preps in several hometowns. There are a couple of areas that also grant reliable access to certain powerful preps. If you don't know what I'm talking about, try asking around in-game. You're bound to find someone who will show you some secret stuff.

Even so, I'm lazy and my warriors almost never rock the more rare and powerful preps that others seem to. Strategy usually beats prepping, within reason.