I know it's nothing compared to playing real MP games, but when you're temporarily disconnected (vacation?) this is the best way to practice, coupled with a good supply of reps.
Practicing your Build Order
The opening is far more generic than the midgame or endgame, so build orders should just be repeated, common knowledge to most. Start out with a BO that matches your playstyle and watch some VODs to see what you missed and where to improve. Chill has some good ones on youtube just search "Zv_ strategy". Or "_v_", really, there's pros for every race. Remember, the best practice exclusively for BO comes from Operation CWAL and Show Me the Money.
Anyway, take this knowledge and pit it against the AIs while keeping your BO consistent. It gets easier over time. Just hang a paper over your computer or something, whatever helps you remember.
Skirmishes and Gameplay
Okay, so you've got your BO settled, yes? What you gotta do is decide what race you want to practice on, and get some of the hardest campaign missions and pit against the AI with very low resources. Now, the good part about the campaign is that you still have to keep your BO cause usually you get barely any army, only about 4-6 workers, and basic structures. The bad is that you can't really practice your opening, so stick to custom games for that.
Recommended Practice Missions
Terrans: Aside from T10, there is no practice in original SC because the Terrans are the introductory race.
TvTSC: Terran10 (The Hammer Falls)
TvTBW: xTerran5b (Birds of War) (this is BC practice) xTerran6 (Emperor's Flight) can give you TvT and TvP mixed practice
TvZBW: xTerran8 (To Chain the Beast) is an excellent mission.
TvPBW: The rarest matchup, the only mission is xTerran6, which is mixed TvT and TvP.
ZvTSC: None are challenging enough.
ZvTBW: xZerg4 (The Liberation of Korhal), xZerg5(True Colors), which is mixed with ZvP, and xZerg10 (Omega), not really necessary for D players.
ZvZSC: Zerg7 (The Culling) is all I could find.
ZvZBW: xZerg6 (Fury of the Swarm) is pretty much it, but it's still pretty good.
ZvPSC: Zerg10 (The invasion of Aiur)
ZvPBW: either xZerg7 (The Drawing of the Web) for practice with ground units, xZerg9 (The Reckoning) for a time limit, or xZerg10 (Omega), the ultimate Zerg experience.
PvTSC: Really rare, I don't think such a map exists in campaign.
PvTBW: xProtoss5 is the best I could find.
PvZSC: Protoss9 for normal practice, or Protoss10 (Eye of the Storm) for the hardest mission in original SC. Really encourages expos and big armies. Not to mention that the zerg covers 90% of the map.
PvZBW: xProtoss8 (Countdown), a good holdout mission with lots of zerg.
PvPSC: Nah. Protoss7 is the hardest, but it's fairly easy. If you're desperate enough, then full steam ahead.
PvPBW: Putting an x in front of the mission, now that's good! It teaches the value of Dark Archons and all in all, killing all three Aldarises will be the best practice.
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Once again, it's nothing like the real thing, but if you ain't connected, then these will fill up your practice time.