I would say that the best tip in order to increase someone's win percentage is be smart. You do not at all have to be a great pvp'er. Hell, I can go on sixzoseven's server and die to just about anyone consistently. But I have a high win percentage (originally 4th highest in us, back in late v1) because I am smart, and I have a lot of knowledge for the game.
The biggest mistake in MCSG by many players, is timing. If you don't time your attacks correctly, it doesn't matter how skilled you are at pvp, I can almost guarantee you will die 9/10 times. What I mean by this is knowing when to attack a team, or knowing when to make that deadly turnaround bowshot or flint and steel lighting. This even includes when to use the fishing rod. And this especially includes when to make the risky move.
As for the pvp aspect, always have 1 secondary weapon that you have mastered. You don't even have to be good with the sword, heck, I'm not the best with a sword, but if you can master at least one secondary such as the bow, you have a much greater chance of killing someone, rather than being okay at all secondary's. A good scenario for this tip would be if you are fighting someone who has better stuff than you, but you have more secondary's. They charge you and you might light him on fire (rather than a guarantee/80-90% chance), and you might get a bowshot but it's unlikely, and you might stun them with the rod. If you master at least one secondary, it is now highly likely that you will get them with whatever secondary you have mastered, thus greatly inreasing your chances of getting the extra hit on someone.
These are just some basic tips, other tips include knowing when to use special tactics for each weapon (block hitting, bow spam, 180 bowshots, flint and steel frenzy, etc.).