I don't think there is a way to "make" your spray always go on top of another spray. From what I've read and experienced, I think it is determined by the order in which sprays are first sprayed during a match. So for example, the map starts and you spray your spray. Someone else comes up and sprays theirs on top of yours. Theirs will go on top of yours. If they spray theirs somewhere else and you try to spray on top, yours will go under it. I hope that makes sense. Basically if you want your spray to go on top, wait as long as you can to spray it (I think the server assigns it a number the first time you spray it and that will determine whos goes on top of whos).