Haste. As an archer, you should be getting full attacks almost every round.
Not a wizard spell, but Blessed Aim is Cleric 1, from Spell Compendium. Similar to Bless, but for ranged attacks only, and a +2 bonus.
Any spell that adds energy damage to your attacks. I think Sonic Weapon from Spell Compendium works on bows, for +1d6 sonic damage per shot.
Greater Invisibility: Best with SA dice, but even w/o, good to-hit benefits.
Targetting Ray, Spell Compendium Wiz 1. To both make it easier to hit, and know where the foe is at all times if he goes invisible or something. Obviously, does not combine well with invisibility spells of your own (as the light shines between you and the target as a line).
Flame Arrow was already mentioned, but also remember you can combine that spell, since it has an energy subtype, with Energy Substitution, to get Cold, Acid, or Electric arrows, depending on which feat version you take.
EDIT: Can't believe I forgot Guided Arrow, also level 1 SpC.
And I notice you mentioned items. The Force enhancement for bows at +2 price is a very good deal, IMO (from MIC). Strong Arm Bracers (MIC again) let you use the next size bow, which means you'd go from d8 base damage to 2d6 (longbow, I assume), and only cost 6000 gp, eventually becoming a steal. On that note, Enlarge Person is a worthwhile spell. Reduces your attack bonus, but if hitting isn't the problem as much as damage (IME, that's the case for archers) you should keep scrolls/wand of it handy. Weapon crystals (MIC once more) should be lookd into. Finally, I can't recall the name or precisely what it did, but i recall a ~10,000 gp quiver in MIC that let you draw various kinds of arrows. Might be too much money for what it offers, though.