These are my favorite tips and I highly recommend you try out all of them - Alt+spacebar+dragging the cursor will rotate your canvas. - Hold the spacebar and drag your cursor to move around the canvas - Hold Ctrl+Alt, and you can change your brush size without having to go back to the brush panel - If you go to View > New (It's on the top menu) and you can have another instance of your file open (Example) - If you click a layer thumbnail and hold Shift, and it will select everything on that layer. - Layer clipping! Color in all your bases, and then keep all the messy shading on a separate layer and click the "clipping group" option, and it will all neatly erase it self outside the lines - Press H to flip your image. This makes it easier to weed out mistakes. - It makes your life a lot easier if you organize your layers into folders!!!
For actual painting tips - Try to keep shading subtle! Don't go overboard with details because more does not always mean good. - I highly recommend having another copy of your picture open (Refer to the New>View tip I told you about) so you can always see the big picture. It might look good up close, but it looks like crap from far away. If you can always see the whole picture, you can make sure it always looks good. - Composition is super important! Seriously I cannot stress this enough. This ties in directly with the tip above. Sometimes you can have a fantastic drawing, but the composition completely ruins it. I have seen a lot of fantastic drawings that get dragged down by composition. Here's a really helpful list of composition links to help you get started. Seriously. COMPOSITION IS SO IMPORTANT. It gets overlooked so much that it is legitimately saddening.