Have your Fighter take a few ranks of Perform (Lute)?
Seriously...
1) You can mix in levels of Ftr, Rgr, Barb, PsiWar, Soulknife, Lurk, Incarnate, Swashbuckler, Beguiler, or Duskblade.
2) As a straight bard, get a mithril Breastplate, Chainmail or Chain Shirt. Also consider the advantages of armor from other sources, like the Datsana or Ashigaru armor from Oriental Adventures.
3) Add a good ranged weapon to your repetoir- hurt 'em first so they can't hurt you later.
4) Use a race with some kind of martial benefit- Elves with their WPs, Dwarves & other races with Weapon Familiarities, Lizardfolk, Warforged, Thri-Kreen & Skarn with their natural weapons and/or Natural Armor, Githzerai & certain Anthropomorphic Animals with unusually high Dex mods (+4 or greater).
5) A trick that is pretty good for boosting any PC's damage output is "going big" using weapons that are larger than normal. There are 3 common ways to do that: Using the Enlarge Person spell; using the Expansion psionic power (like Enlarge Person, but good for 2 size bumps) or supersizing your weapon via Monkey Grip or a race with Powerful Build.
6) Careful Feat selection is your buddy. High Dex? Combat Reflexes and Deft Opportunist can be fun. High Str? TWF + Oversized TWF can boost your damage output. If you're playing in Faerun, use a longsword and have it made of Gold or Platinum for the Heavy Weapon Feat (Magic of Faerun) to boost its base damage.
7) Mix in levels of a class that can use spells that boost martial performance- like Sorcerer, for instance- and flog those spells. A personal fave of mine is Whirling Blade (esp. with Sculpt Spell) combined with any 2-handed bladed weapon, but the Bard's Longsword is good enough to make the spell a good mook-slayer.
8) Your bard probably has a high Dex- take feats like Combat Reflexes, Expert Tactitian and Deft Opportunist. This will work especially well with his sole standard reach weapon, the Longspear...which, BTW, does just as much damage as some Martial weapons.