Some online swaps are ongoing, some just for a month or two. Last year's birthday block swaps were great for a couple months, then folks dropped out until us winter born folks got shorted.
Before you join a swap, make sure you either know you can rely on these people in it, or can afford to "donate" blocks to people who may not reciprocate.
I'd look for swaps where a moderator collects the blocks, along with return postage, then forwards them out on a given deadline.
By the way, US folks, postage is going up again - to 44 cents for the first ounce, so buy those forever stamps now!