pick a replay with a solid build and study it inside and out.
practice that build over and over against, against a computer if you want, so you dont miss depots, you dont miss factories
honestly, I spent 3 or 4 days just going over one build over and over again, making sure I could do it without a hitch pretty much every time, that I could adapt it fairly well, and it was probably the single greatest jump in my starcraft abilities.
I would reccomend replays involving koreans though