Let's start with the basics. If your bathroom has an overhead ceiling fan, make sure it is on. Stuff a towel under the door to keep any smell from escaping the room. If you have a window in your bathroom, open it. Turn on the bath/shower and leave this running to hide any sound of the lighter, you coughing, etc. Also, once you're done smoking you can take a bath or shower to relax. It's something you can definitely enjoy. And last but not least, make sure you have some sort of air freshener/perfume/etc. to conceal any lingering smell, but be sure not to spray too much to make it obvious you are hiding something.
This is basically all you need to know. If you are feeling extra paranoid, make a sploof and don't smoke any blunts/joints in the bathroom because they produce a very strong smell because of the paper. If you have a one-hitter, use that.