Ok, then what you wanna do is get the rest off. Paint detailers use this thing made called a Gum Rubber Wheel, and its basically a 5-inch diameter rubber 'eraser' used to remove stickers and decals and other gummy, sticky stuff. Or, if your planning on only doing this once, there's a product called Lift Off. It comes in a spray can and what it does is after you spray it on and wait however long, it'll penetrate the glue holding the stripe on and seperate it from the clear. The stripe should wipe right off, but it shouldn't hurt the clear in anyway, no guarantee on polishes or waxes or anything though. I've seen this stuff round places like Lowes/Home Depot. It should only take about 10 to 15 minutes and then your ready to wash your car!
You could put painters tape on there to give you a base line, it would certainly be helpful in that if you make any mistakes its not gonna be that big a deal since its not your final app. When I said straight though I didn't mean like level or anything, its straight as in following the body lines of your truck. The wrinkling shouldn't happen unless you've got it goin at a pretty extreme angle as far as straightness is concerned, but if you don't keep a measure of taughtness on the stripe when its applied then it could crinkle up. Then again though you don't want to go all hercules on it, its not a rope or anything haha. You want just enough to keep it from binding up on itself. What might even be a good idea for you is after your application area is dry enough, take masking tape or painters tape or something and mark first the beginning and ending marks of the stripe, and then with a pencil, mark on the tape the level that you want the center of the stripe to correspond to. Then you might wanna run your masking tape length wise either below or above the stripe, making adjustments along the way. ALSO as you apply the stripe DON'T run your fingers along where its already made contact with the car, make sure you hold it just ahead of the contact area, this makes sure you don't stretch the material too much or even rip it (OUCH)