Note that non-Swordsage levels only increase your initiator level (by 1/2 lvls). They don't give you more maneuvers or anything like that.
Best one-level dip: Cleric. Gain a few spells, two domains (that can easily be two good feats, or any two of lots of special powers), access to spell trigger and spell completion items using Cleric spells (that's a lot right there), heavy armor proficiency, +2 to your two most important saves.
Or, depending on your preferences, Cloistered Cleric has a weaker HD and only light armor prof, but lots of skillpoints, plus a third domain (Knowledge) and access to a few divination spells.
Other good dips:
Warblade - superior recharge mechanic compared to Swordsage, +1 BAB, 1d12 HD and all around good stuff. Actually, dip Warblade as much as you like...
Conjurer is also a nice one-level dip even for warrior types, for the Abrupt Jaunt ACF and access to (self-buff) spell trigger/completion items.
A level of Monk is actually pretty nice, with all good saves, two bonus feats (Improved Unarmed Strike plus one other, depending on which variant you get), and Wis to AC (unarmored).