Talk with your players beforehand, so they know what you expect of them (and what they can expect of you) in terms of post rates.
Combat can be tricky, since there are times when players may want to go full defense when being attacked, and unless everyone is committed to posting at least 1/day, then posting in initiative order can bog things down. If you have to, simplify initiatives so that it is before baddies, baddies, after baddies in each pass. In the baddy post, sum up the effects of everything that has happened since the last baddy post, and if people are attacking your players, post the number of hits they got on the attack, and the damage. Let the player decide whether to abort to full defense or not. So you have Baddy A swinging at Player 1 with his katana. He gets 4 hits on the attack, and has a 3 strength. In your post (or if you have an OOC thread for actions and stuff, put it there) write: "Baddy A attacks Player 1 with his katana. 4 hits, 5P+net hits DV, -1 AP." Does this give away some of the stats of your baddies? Sure, but you gotta do something to keep the game from stalling.
Where PBP shadowrun games really stall is in the planning stage. Every so often give them a nudge in the OOC, asking if they are ready to move forward.