Make sure you're building units out of your production facilities

If you have enough production facilities, but are not producing units out of them, they are worthless buildings. Make sure that all of your production facilities are building units!

Spending Resources

Spending Resources is one of the primary objectives in the game. If you have a bunch of unspent resources there is normally a few causes.

• You are not producing units out of your production facilities. Make sure when you have a warp in cycle that you are producing units.
• You are getting supply blocked too much
• You do not have enough production facilities, make sure you have enough to make your current income level.

Built too much supply ahead of time

Overproducing pylons to early in the game cuts into your resources that could be better spent on units, workers, buildings (non- pylons). Work on only staying about 10-15 supply over your currently supply. When you progress to the later stages of the game start shooting for 20-25 over your supply, this should allow you to warp in without getting supply blocked.

Spending Gas as Zerg / Terran / Protoss

If you have lots of Vespene as Protoss, one of the best ways to use it all is to make archons out of High Templar or Dark Templar, It's better to make them out of High Templar since it’s cheaper on the minerals, but if you already have a Dark Shrine making them out of DT’s is fine too.