CS is the most inconsistent game for me. I will go 10-0 on start of one map and 0-10 on the following. I will walk around and insta HS everyone I see and they don't even really react, everything is perfect and they're turned the wrong way at the right time. And other times, I barely see someone and get insta hs'd over and over, or through smoke, or through walls, or walk into flash after flash.
What I do if I'm doing poorly is switch weapons. I often scout/deagle, but if my aim is just shit that day and nothing is working out, I go with an M4/AK. If that doesn't work, I get a p90, mac10, ump, whatever. I just dick around at that point until I start getting kills again.
Also switching your style of play can help. If you usually camp back and keep getting killed, buy a p90 and rush mid on dust2 on either team all crazy.