In addition to Doug’s suggestions, I have some more:
Unearthed Arcana flaws
The option of taking Flaws is presented in Unearthed Arcana. For each flaw a character has (up to two), the character gains an additional feat.
In general, this is a great trade every time; you can easily take flaws that do not impact your character much, or match behaviors you were exhibiting anyway for the sake of characterization, or simply aren’t that burdensome. This is generally seen as a problem with the Flaw system’s design, but many DMs allow it anyway, as an easy way to strap more feats onto characters, while encouraging at least a little thought about player character’s flaws.
The other nice thing is that it makes humans less of a “gimme.” Doubling your 1st-level feats is huge; it’s a 100% improvement. Adding a fourth feat when you already have three is only a 33% improvement; less critical. Humans still remain very popular, even with flaws, since feats are just that valuable, but flaws make it much more likely that someone will find another race’s offerings to be more valuable than a fourth feat.
The advantage of this, as opposed to something like Pathfinder’s feat-at-odd-levels, is that you get the additional feats up-front. While Pathfinder offers four more feats than 3.5, these feats do not show up until 7th level; at the levels your players are at, they’d see only a single additional feat. You generally take Flaws as a part of character creation; this helps you get prerequisite feats out of the way, and in my experience, works better because of it.
Of course, you can do both; I’ve played in many successful games where we had both flaws and feats-every-odd-level. With extreme number of feat requirements for things in 3.5, this has never really been a problem, and has meant that players were more willing and able to pursue options that ordinarily just take too many feats.
Feat tax evasion
Another thing is that 3.5 is utterly plagued by so-called “feat taxes.” These are weak feats, or at the very least feats you wouldn’t otherwise take, that you end up having to take in order to take the feats or prestige classes you do want. There has probably never been a 3.5 character that hasn’t run into this problem.
Craft Wondrous Item and Craft Magical Arms and Armor as requirements for Craft Construct are excellent examples: neither’s really a bad feat, and you can kind of see where they were coming from using them as requirements, but ultimately they just are not really necessary here. Golems only have a passing resemblance to weapons and armor, really; it’s not like you’re making a flaming keen golem or a light fortitude golem. So nix the requirement.
You can do this all over the place. I have played in many games which eliminated Combat Casting, Combat Expertise, Dodge, Point-blank Shot, and/or Weapon Focus from the game entirely, or at least from requirements, and in my opinion that massively improved things. These are very-weak feats that are required for a ton of different options. When you eliminate these feats, you allow players to more easily take more interesting feats, allowing them to get benefits in keeping with the relative scarcity (even with flaws and/or feats-at-odd-levels) of feats.
Kill the “There’s a Feat For That!” Phenomenon
Part of the problem with 3.5’s extremely large selection of feats is that you start to get the idea that you can only do things that you have feats for. Especially when really basic things start appearing as feats – my personal pet peeve in this regard is Spell Thematics: why does a wizard need a feat to say that his fireball looks like a giant flaming skull? Does that matter? No, it’s just a cool bit of characterization; that sort of thing should be encouraged, not charged a feat for.
The authors themselves seemed to go back and forth on this one; some books (or even some sections of books) encourage players to be clever and DMs to allow them to do things differently, while others print feats that imply the feat is necessary.
This doesn’t really apply to Craft Construct; that’s probably a pretty reasonable feat. But this is something to consider when you talk to players about feats they want, are taking, or are considering: you may be able to considerably improve their experience just by looking at a feat and saying “you don’t need a feat to do this, just say you’re doing it.”