General Generally speaking, your best bet in the early to mid game is to island hop. If at all possible, take roundabout routes to whatever island you are aiming for as to avoid any corsairs or wraiths that might be looking to intercept you. It's sometimes a good idea to bring some empty overlords along as distractions. Well now comes the hard part, protecting your expansion. Hydras are great.
Lurkers are even better.
A combination of these two units will usually do a very good job at stopping any drops on your expansion. While your new hatchery is building, you're probably getting high on minerals. If you're feeling risky, grab another expansion. If your opponent has already spotted your first expansion, you may just want to pump out more units and start teching. Now that you have an idea of the zerg's role on island maps, now lets look at specific match ups.