Plan to travel light, especially if travel plans call for domestic flights, where there’s a severe weight restriction on luggage. Launderettes aren’t hard to come by once clean clothes are exhausted.
Most parts of Panama  are hot and humid year-round. Bring lots of thin cotton or breathable, moisture-wicking synthetic clothing. Keep in mind that dress in Panama is conservative.
It can cool off significantly in some regions of Panama, and air-conditioning can raise goose bumps in the lowland cities. A light sweater or fleece along with a waterproof windbreaker or parka is a useful combination. Consider taking along a small umbrella for use in town.
If you think you’ll spend much time island-hopping, consider bringing a life jacket. These are often not standard equipment.
Consider bringing a bedroll or light sleeping bag even if you’re not planning on doing serious camping. It can come in handy if you have to spend a night in a place without decent linens. And don’t forget the binoculars.