So treat guards the way their employers do: like furniture. Get them out of your way with a minimum of fuss, and realize that jumping and climbing on them is childish. There's no shame in carving your way through a squad if that's what needs doing. But that sort of thing is wasteful if done unnecessary.
Slitting throats works if you truly must, but slamming a helmeted head against a wall is much less messy and doesn't make much more noise. Bind 'em, gag 'em, stick 'em in a corner somewhere and go about your business.
If you must steal a uniform, steal one from a bigger guard. Might put up more of a fight, but it's much easier to strap down a big uniform than move around in a tight one.
Ladies, don't bother seducing one if you're already inside the walls, no matter how flattering your armor is. Guards aren't the brightest torches in the hall, but most are swift enough to realize that any henchmen fanciers wandering by would find quite willing companions at the first guard shack, long before getting into the fortress proper. Save such gambit for city watches. Much more effective there.