You can think of images that excite you, relive a specific event or think of a person that stimulates your desire.
My first time masterbating
Or, use something more explicit like pornographic films. Different positions, different areas to stimulate, and many ways to self explore. If you find you hand tiring, then you can try using a dildo instead. Mmm you're fuckin sexy baby you've got my cock so hard right now. That was hot, just wish I was between your legs licking your juicy hot horny pussy. I cover more about anal sex in this guide.