When starting the mobile app (UI/UX) designs make sure you keep the user in mind. The whole point of all this is to make the users life easier or benefit them by using the app. Making the registration process streamlined or keeping things out of sight they won't use often are a few examples of that.
Adding time or complications to the user aren't things designers or developers try to do. Get others that aren't working on the design to see if the work flow makes sense or if it seems overly complex. Sometimes the solution is by making the process longer, but simplified overall. Going back to the registration example. Most newer apps ask you one question at a time when creating a profile. This keeps the user on task and doesn't overwhelm them. This allows for them to simply answer the question and move on. The user feels like it is a speedy process and makes it easy.
I stress this at the beginning because there are so many apps that designed because the app needs to do x, y, and z and it wasn't thought about how natural this is going to be for the user.
Going into your design make sure that you look into trends and competitors. You want to stand out from the competitors but be on the forefront of the design trends.
The details are extremely important. How a button works or is placed can really effect how efficiently the app is being used. Navigation is the key component in this. It is going to get users around the app. Typography is a necessity in this as you have limited space in your layout and need the type to be user friendly. The color scheme, this seems odd to pre pick out color but helps you limit your colors in your design. This allows you for you to see how the colors work together as well.
Quality control is a must. You want to make sure your app works and in the way you intended. This is also you chances to make sure it works well for the user. Checking to make sure buttons are in reach of your fingers of your right hand. Most people use their phone one handed. Also try to check and make sure you app looks good on older models. You want everyone to have the same great experience.
Some final advice, make sure you work closely with your developers throughout this process. You want to make sure your vision can me created and they know how to. You don't want to create this amazing idea and have no way for it to be created.