This tip seems pretty straightforward, but you'd be surprised by how many people ask me why they're not gaining weight when their only carbohydrate sources come from vegetables, trace amounts of sugars, fruits, and legumes.
I'm not saying to go completely crazy on trashy carbohydrates, but your body will gain better results from additional carb sources such as rice, oats, sweet potatoes, and—dare I say it—bread. This is especially true with heavy weightlifting, since carbs are needed to replenish glycogen stores that a particularly grueling lifting session devours. Some studies suggest that timing the majority of your starches around when you train may shunt unnecessary fat storage. For example, eat these starches either pre- or post-workout.