First off, teleporting. An attentive enemy might notice the sign of someone warping in, but even if they do, they won't know who and in what condition you're at.
Second, play fog of war to your advantage. If you see a teammate fighting someone they might not be able to see you (depending on where you are), and you can either come in from the side, or cut them off then they run.
Third, and sneakiest, hide behind a larger character. Rook works wonders for this. If you fallow closely behind, your opponent might not know he's in for a 2v1 until he's suddenly at half health for no reason.
#1 used to be really effective because the port animation didn't exist. Still always fun to watch some poor new guy scratching his head when he sees three whirlpools pop up at the flag he is trying to cap
#2 is really important and speaks to the need for map awareness and I would add that you want to position your approach within the fog to best advantage - if you can cut off his line of retreat before he sees you then he's in alot of trouble.
#3 can be really useful for foiling enemy abilities and achieving stealth kills - you don't even need a Rook. Sedna and Oak can walk inside each other to confuse their profiles, for example. Huddling with allies, minions, or reinforcement waves is good practice as long as your enemy doesn't lead with AoE. Also, you can't really predict your enemies' POV but I've definitely gotten a stealth kill or two by sidling up close to towers, tower connecting walls, or citadels too...