Again, it's just practice, practice, practice. After you've written orders for the same meds over and over again, you'll have those dosages burned into your memory.
For example, in G Surg, we write for the following meds all the time:
Pain meds:
Percocet, Vicodin, Lortab, Dilaudid, Toradol
ABX:
Ancef, Keflex, Rocephin, Vancomycin, Levaquin, Flagyl, Zosyn, Bactrim
GI:
Colace, Dulcolax, Fleets, GoLytely, Nexium, Pepcid, Protonix, Phenergan, Zofran
DVT:
Heparin, Lovenox
Etc.