In real life poker, if you stand up or leave before your blind, and then sit back down, either in a different seat or back at the same one, you have an option to either post the blinds and be dealt in or to wait until the blinds hit you again, and then post them and be dealt in, you cant get out of paying blinds just by getting up from a table.
Same goes for new players joining a table, if they want, they can post blinds and get dealt in straight away, or they can just wait for the normal action to get to them.
Tournament play is a bit different.... you never really leave a table until you have no money left, so if you are there or not there, you post the blinds in turn, and you are always dealt cards, and you are check folded from there... so if it comes your play with no raises in front of you, you automatically check, and if there have been a raise, you are automatically folded.
(sorry, i was writing this when bobwhoosta was posting his)