I think sieges have become slightly more common in 0.43.05 than it was in 0.40.X as the result of fixing an army pathing bug (but resulting in various siege bugs currently, such as hello/goodbye sieges and siegers caught in limbo).
Embarking near a black tower gives you a chance for a 2-3 sieges, after which they run out of bodies (provided you don't just turtle until the go away, of course). However, you should be the closest target for the tower: I've had a number of cases where I FINALLY managed to get a tower within range, only to see (through Legends Mode) that they immediately went to war with 5 other civs and were decimated.
Ensuring sieges from goblins is similarly finicky: You should be their closest target, or they're likely to deal with other targets first. More importantly, their closest site should be a well stocked one, as armies are sent from a single site only and DF doesn't know how to recruit from other sites yet, so a 50 pop black pit is probably good for 2-3 sieges. 70 years later you may get two insult sieges of raw recruits when they've finally climbed back up to a pop of 30 (has happened to me). Thus, make sure their closest site is either their dark tower or a large site (again, Legends Mode is useful here to check that the world actually may be useful).
Since the next release can provoke wars with civs that aren't currently eligible because they're not the closest one for their race, new options might appear then (whenever then is, X months away).
Derpy Dev is correct in that the default siege trigger is a pop of 80, but it can be lowered to 50 or 20. snow dwarf's comment about necros is correct as well: they can show up in the first summer, so you should have a door ready and sufficient stores inside by then.