I was there for a week last year and really enjoyed it, except for the food poisoning. First thing i agree with the above, dont get your passport stamped we arranged this at mexico city airport on the day that we left to fly out to cuba. you have to pay $30 USD for your visa. Learn basic spanish.... very few people speak english in cuba. there are no atm's in cuba (that i am aware of) get all your money out prior to leaving mexico (or where ever your leaving from) dont take US dollars with you, they dont except it, we took canadian so make sure you exchange this prior to leaving. be prepared for a very limited source of food. Cuba does not import from the US, only from mexico, canada and china so fruit and veg is limited. supermarkets have alot of bare shelves, grab what you can when you can ie snacks and muesli bars. I found they only have one variety or brand of produce. Remember to tip when you eat out, i think its up to 15%. remember there are two types of currencies for the convertible peso, you being a tourist will always be charged more. the only places were you can get more money out is from some hotels. Havana airport is sooo strange, going through customs is like going through a random double door... pick a door any orange door
stay safe, alot of people say Havana can be rough, it is a night so remember to taxi it back to your hotel.
for the sake of travelling back to the US during your travels, i sent everything i brought via fed ex, I just cant deal with US customs and explaining where the hell i got my cuban cigars.
if you are buying any cuban cigars, make sure you buy from a government owned tobacco store that has the sealed gov stamp, if you just buy off any random joe off the street your in deep shit if you get caught. this goes with art work aswell - all has to have a gov stamp on the back.
One of my favourite moments in cuba was just walking to the local store in Havana, randomly there were 6 people dancing on the side walk and they invited my friend and i to dance. such fun
and above all have a decent Cuba Libre (rum/cola) there is nothing more sweeter than cuban rum.
Be prepared to have an open mind about cuba, they have only really just opened up their doors to tourists in the last few years and only just got mobile phones in the last 1 1/2 yrs. so technology is limited. there is no internet - only at the hotels and its very expensive, no wireless so forget your laptop, no cable (like US and MEX) only 3 channels. the place is just amazing if you like history, museums, under ground clubs for dance and awesome jazz bands and CHE, old american cars and run down mafia hotels. I hope you enjoy cuba as much as i did, at first i was thinking why did i come here, but after a few days get your barings and enjoy cuba for what is has to offer.
Hope this helps, sorry ive ranted on, and could continue - if you have any other questions feel free to ask