Abdulla, here's how I do it:
1) Let car sit in the sun or heat up the spoiler with a hairdryer or heatgun (be careful with the heatgun)
2) Spray "goo gone" as much as you can between the spoiler and the trunk. let sit for 15-30 minutes, go eat or something.
3) take some fishing line, and start at one end, and gently "saw" across the trunk. You'll use the fishing line to "cut" through all the tape.
4) Once cut all the way through, you'll be able to gently pry it up and get it to unstick.
5) For any remaining tape, you can get a microfiber towel and use some rubbing alcohol. Pour the alcohol on the remaining tape and "rub" against it with the towel. You can also use your thumbs or fingers (but blistering can be an issue). If you have a LOT of tape left, you can go buy a rubberwheel bit for a dremel or rotary tool. You can use the dremel with this bit and use it like a eraser to take off the tape. Be careful because you don't want to mess up your trunk. The dremel method is by far the fastest and most painless way.