There is a dangerous lack of basic medical supplies in Cuba. Hospitals find themselves overstaffed (the medical education in Cuba is top notch) as their are more doctors than medicine. Items that we take for granted, band aids, condoms, asprin, are not readily availible on the island.
Keep this in mind when packing for your trip. Giving out extra medical supplies is as valuable as giving out money. I found it best to do this discreetly. Approaching someone on the street and handing them a box of condoms may not be a great idea (just imagine yourself on the recieving end in your home). If you are staying in a hotel, when you check out, leave the supplies on your bed. The staff will much appreciate this, believe me. Or say to someone "I have some extra supplies that i don't want to lug back home in my suitcase. Do you know anyone who could use these?" This will make your deed seem less condescending.
Also, tip as often as possible. There's no need to go over board with the amount. Dollars go a long way in Cuba. It is a responsibity of every traveler to Cuba to give a little back. Remember, the very act of getting on an airplane and leaving your country is a priveledge most Cubans are denied. This is a very poor country with a great deal to offer the world. It will go a long way to show some appreciation and support for the people you will meet.