you do need a slip of paper given to you by your hotel, however think they are not as keen on this anymore as heard of a few people who did not have it and were allowed to fly with no problems, but best be on the safe side - the hotel take your passport for a few days and when you get it back it will have the slip in it.
It depends on how often you go / how long you are staying
According to the Bulgarian embassy website
(http://www.bulgarianembassy.org.uk/visa/General%20Visa%20Information%20for%20Bulgaria.htm)
" Holders of valid ordinary passports from the following countries do not need a visa to visit Bulgaria for up to 30 days within each period of six months:
"Andorra, Australia, Canada, Estonia, Ireland, Israel, Liechtenstein, Monaco, New Zealand, Switzerland, UK, USA, the Vatican. "