Tricks to archery in 3.5:
1) Stacking bonuses: If you have a +1 Flaming Composite Bow, and you use +1 Frost Arrows, you fire +1 Flaming Frost attacks. Have someone cast (or use a wand) of Greater Magic Weapon cast at 8th level, and now you fire +2 Flaming Frost attacks. Greater Magic Weapon
scales all the way up to 20th level (+5 bonus), so you should never buy any bow or arrow with anything but +1 to its enhancement bonus.
2) Splitting Arrows: This +3 enchantment for your bow from Champions of Ruins literally doubles the number of attacks. If possible, get it.
3) Precision damage: This doesn't apply to your build, but precision damage and ability damage (Crippling Strike, Mind Cripple, Maiming Strike, Wounding) can apply to every attack when you have Greater Manyshot. If you don't have precision damage, then you don't need Greater Manyshot. Regular Manyshot and Rapidshot will work fine for you.
4) Green Arrow Syndrome: Have specialty arrows for every occasion. Specifically, you should have arrows made of silver, adamantine, and cold iron, to deal with damage reduction. The various supplements also have blunt arrows, rope cutting arrows, hollow tipped arrows that you can put holy water or acid in, etc. Bane
is also a cheap and efficient enhancement that you should have many varieties of. And be mindful that many enemies have specific types of energy vulnerability.
5) Money Money Money: More then any other build, you depend on your stuff. You'll be firing a lot of arrows, and 50% of those arrows will be useless after you use them. This can be mitigated with Mercantile Background (Player's Guide to Faerun, Forgotten Realms Campaign Setting, Dragon Mag), Ancestral Relic (Book of Exalted Deeds), and high Appraise (to always take the best loot) and Diplomacy (to negotiate better contracts). The Kensai PrC from Complete Warrior also helps with this a lot.
6) Hide in Plain Site: You can use the Hide Skill as part of any Move action, as long as you have appropriate cover. Hide in Plain Site removes the need for cover. Thus you can use Manyshot, then Move and Hide every round. When you are hidden, you cannot be targeted by any attack that requires line of sight (everything but area of effect attacks). Although you'll take the -20 penalty for Sniping, its ridiculously easy to boost your Hide bonus with the right race, magic items, and maybe a 1 level dip in Exemplar. There are a few dozen ways of getting this, including core Ranger, or a dip into Warlock
7) Fear: Fear effects stack, and are very potent against mobs of weak enemies. You'll be making a lot of attacks every round, and you'll usually also have your Move Action available to do other stuff. Add Fear to your attacks with various enchantments, classes, grafts, and/or feats. Or use Intimidate as a Move action every turn to Demoralize your enemies
and then fire on your enemies with Manyshot as a Standard Action. Check out the Frightful Presence feat from the Draconomicon, the dragon grafts and PrC from Races of the Dragon, and the various spells and magic items that inspire fear effects. Don't go overboard with this though, because tons of enemies are also immune to Fear effects.
8) Poison: Another way to add damage to your attacks. This probably doesn't apply to you, because by RAW, poison use is Evil. (Many DMs ignore this - how is Poison more Evil then splitting someone's skull in half?) But my Rangers and Scouts always make a habit of harvesting poison from enemies that use it, and applying them to arrows to be stored for special occasions. You can also take Craft (Poison) and make it yourself between adventures. Book of Exalted Deeds also has rules for "ravages" which are "good poisons" (I don't understand it either).
9) Spells: There are a variety of spells that apply specifically to bow or weapons in general, which can be used to buff you or pull of special effects. Ranger spells in particular from the various supplements help a lot on this. Not sure if this applies to you or not.