Honestly, just practice 1-2 build orders for each matchup until the macro becomes second nature and you'll have considerably more time to worry about microing your army/scouting, etc. Try macroing away from your base only going back to place structures and drop mules, keep your vision on your active units. The more you practice it, the more time you'll have to micro and the more practice you'll get with effectively microing while macroing. It's a simple tip, but the fact is a lot of people can't do both easily. Once you get macro down and dont have to worry about, push your micro beyond what you can do, try doing 3 drops at once and swapping between microing each of them as needed. It'll be hard to do at first, but by forcing your opponent to react they'll be more hard pressed to defend and you'll get practice microing intensively. Even if you're a low apm player, your apm will start to skyrocket in those situations without realizing it with some practice.
There are some multi-tasking trainers out there:
http://www.teamliquid.net/forum/viewmessage.php?topic_id=124983 But generally I would advise to just practice doing a little more than you can handle until you can get a good grasp on it and just gradually increase the amount you're doing at once. By training your mind to handle multiple drops you'll be better prepared to handle the same thing from your opponent and you will have the multi-tasking ability to effectively micro a death ball really quickly
