Multiclassing and Prestiging out
Swordsages don't gain much out of Multiclassing compared to crusaders or Warblades. Although doing it doesn't completely ruin it like multiclassing wizard with fighter thanks to the wonderful thing, 1/2 initiater levels.
I am only presuming 2 level dips for these descriptions unless otherwise noted, nothing long term, but I reccomend taking no more than 6 levels of 1.2 initiator classes if you want a level 9 maneuver pre-epic.
Core classes
Spoiler
Fighter:
A few bonus feats,better HD and Full BAB, which makes no difference unless you take more than 4 levels and remember,
3rd level fighter dumb, fighter level 3 is dumb, thog not take take.
And you need 4 full BAB levels for the 4th iterative attack, or even a better BAB by level 20.
Paladin:
Only Cyan because with an obscure feat from dragon compendium you can get wisdom to all saves or AC, not that good since you are not front line fighter or likely to take part in extended duels with casters.
Rogue:
Only worth taking at 1st level and 8th level with able learner so you can be a pretty good skill-monkey and have extra sneak attack.
If you do this it is a good idea to prestige out at some point into Eternal Blade.
Ranger:
Same skill points and HD, but gives a bonus feat, either something that lets you do something in combat that isn't insert pointy end into opponent.
Complete's
Spoiler
Samurai:
Skip it, it doesn't give anything in small dips paladin doesn't beside a weakened two-weapon fighting and proficiency in Bastard Sword.
Hexblade:
You need 3 levels for any useful return, 1st level gives you a pretty awful curse, second level gives you charisma modifier to saves against spells and spell-like ability's, not that good since it then removes your only true dump stat.
3rd level gives you mettle, which is basically evasion for fortitude and will saves. You will make most Will saves, but presuming you go 3 Hexblade/ 17 Swordsage you will have +6 base fortitude save, you won't be making many saves designed to challenge the parties Warblade or Crusader.
Ninja:
Swordsage is a good dip for Ninja, but ninja is not a good dip for Swordsage.
Scout:
Swordsages move a lot in combat, scouts deal more damage in combat my moving, a small dip isn't bad, but it delays maneuver progression.
Warlock and Dragonfire Adept.