Tip #1: Layers, layers, and more layers. You're probably not going to exhaust your layer options in a single drawing, so make use of them. Layers give you enormous flexibility when it comes to blending, correction, adjustment, and more. It's a lot less painful to clear a layer with some minor edits that it is to undo 300 changes to get back to mistake you want to correct. Note, there is a layer limit and you can see what it currently is by looking at the canvas information in the actions menu under canvas.
Tip #2: The selection tool is your friend. If you learn to combine this tool with underlying layers, you can easly constrain your work to being "inside the lines." So, for example, if you have an underlying base layer and you want to constrain your highlights layer to that region, use the selection tool on the base layer to select a region of interest, then pick your highlight layer and start working. Instant protection from going outside the lines.
Tip #4: Use a reference layer. The reference layer is a very, okay it's an amazingly, handy way of constraining flood fills on subsequent layers based on an underlying image structure. So, think about it this way, you do a basic simple sketch, you create an "inking" layer and now you want to start defining a base colour layer. Using the flood fill is a great way to quickly drop colour in, but if you create a new layer over your inking layer, the flood will fill the whole layer. This is where the reference comes in... On your inking layer in the layer menu, tap to get the layer options and select reference layer. After that, your flood fills on subsequent layers are constrained by the lines in the inking layer. Wickedly useful, designed for manga/comic artists that tend to black outline.
Tip #5: The power of three is very handy... The cherry blossom image above was created as a part of a tutorial I was following and it made extensive use of this tip: use three essential layers. At various points of this drawing, I created individual layers to manage the midtones, shadows, and highlights. What's the trick? Set your shadows to "multiply" in the layer blending mode, and your highlights to "screen." Stack with the midtones on the bottom, shadows next, highlights on top. Use highly controllable brushes, I used the soft airbrush for example, to gradually build up as you need and, well, you start to build the illusion of dimension quite readily.
Tip #6: Group your layers. To group, all you need to do is swipe right each layer you want to group and a menu option to group them will appear at the the top of the layer menu. Once you group, you can rename the group and perform some combined actions on them. Combining this with Tip 5 allows you to create focused regions of change.That's 6 tips around layers and you can see, fairly quickly, how powerful a tool that function is for the digital artist. Layers are an effective "must have" for any serious graphics/drawing/editing program and it's easy to see why.
Bonus Tip: Palettes are your friend. Building your palette from a reference image is very handy, but make use of these as a means of being able to revisit certain colours later in your drawing even if you're pulling colours directly from the colour wheel. You can even save palettes for later, in Dropbox for example, and bring them back into the app later if you need.