PrCs:
Wyrm Wizard and Recaster add a very few spells known, although I can't recall off the top of my head whether a Duskblade will have a very hard or merely a hard time qualifying.
Mage of the Arcane Order would be much, much better, superficially speaking. A bit hard to get into, but the reward is "any arcane spell of a level you can cast is now available to you". Quite good, even though a Duskblade may lose a bit too much focus going this route. Better for Sorcerers, to be honest.
Rainbow Servant's capstone opens up all Cleric spells for you. Requires similar betrayal of typical Duskblade ideals (which most of the time revolve around crushing your enemies, seeing them driven before you, and hearing the lamentation of their women IME).
Technically, Ultimate Magus allows you to shift over spells from your spellbook into your spells known. Duskblade Magi are rather uncommon though, and for good reason.
Feat:
Nobody mentioned this yet? Extra Spell lets you choose a spell you do not know to add to your spells known list, up to a level lower than the highest level you can cast. An argument can be made that this includes spells off any list, including those not on your class's spell list. Interpretation of this feat is up to your DM, so handle with care.
Item:
Runestaves are a lovely option. They allow you to use a spell slot to cast a spell you do not know of the same level, predetermined by the Runestaff you have. Basically, they add spells known.
What has come before:
Mother Cyst is icky, and eeeeeeeevil, but effective. Recommended.
Arcane Disciple may be good circumstantially, but you invest a lot into just one or two nice new tricks: you invest a precious feat slot (Duskblades are very feat-strapped); your Wis needs to be high enough to cast the spell, making you MAD; and you only get one casting per day. I don't like it that much.
A Sandshaper dip is awesome, especially since it includes very nice self-buffs and makes them spammable (low level).
Can't comment on the other options mentioned by Stream.