Finding inspiration can be difficult, especially if you're doing personal projects. The nice thing about client work is that you have some parameters that you have to meet. If the client's app is a messaging app that helps define what your iconography should be, though it's still a good idea to go above and beyond and try to develop something truly creative. If you're doing a personal project however, the sky is the limit.
Before you have any client work, or if you just want to hone your skills you are usually presented with a blank page. It's intimidating, but giving yourself a goal is a great way to make things a little easier. Rather than a range of limitless possibilities you have a problem to solve. It doesn't have to be a big problem, but it should be something that's going to inspire you.
A great way to find problems that you could solve is to look at problems that have already been solved. This sounds counter-intuitive but there are a lot of solutions out there that aren't well thought out. One of my favorite ways to get inspiration is to browse the App Store and find icons that don't really solve the problem they were designed to. In addition, it's a great exercise in identifying what does and doesn't work and why. I am however against outright copying, the idea of re-solving problems is to give your own take on a solution. Your product should be a different take on your inspiration.