LOL, I really don't have any trade secrets with applique... yet.

Keep in mind I've only been doing applique beyond simple squares & hearts since last December and have only used quilter weight cotton for the applique (so far). That said, I have been sewing for 30+ years so that definitely helped the learning curve.

Here are some things that I have learned along the way:
* One thing that has really helped me is a cool notion called a pressing sheet. It allows me to iron the pieces together as I assemble them before they are attached to the base garment/fabric. The pieces peel off easily when cool. Once I peel it off, I can check to be sure the pieces have overlapped where needed so there are no gaps where the fabric might fray. If there are gaps, I peel those pieces apart (very carefully) and reposition to overlap correctly.
* I'll mention again using prints (marbles, tonals, etc.) where possible instead of solids. Even the nice Kona cotton fabric from Kaufman tends to fray more easily. When I look for fabric to use for applique, I look for tonals in smallish prints and I also often check the back. Sometimes the reverse of the fabric will work better as a replacement for a solid than the print side. It also, IMO, makes the applique more interesting if there's some variation in color, pattern, etc. within the applique. For example in this one, the only solid is the fleshtone.
* Match thread to the applique fabric. This means I may change my thread (top only since I use bobbin thread for applique) 10-20 times on an appliqued item depending on how many colors is in the applique and how the pieces overlap. I try to use thread that is slightly darker than the fabric so it helps act as an outline. I tried using black to outline all areas of Mickey & Minnie but I didn't like how I could see a little of the fabric through the stitching even when my stitches were as close together as my machine would do. By using thread closer in color to the fabric, the stitching looks smoother IMO. Below are examples of similar Minnies - the one with black stitching is one of the first detailed appliques I did. I think it's decent but I was bothered by how the fabric showed through the stitching. The other Minnie is one I did recently and matched thread to fabric. My stitching still isn't perfect but my mistakes aren't as noticable.
* Practice! When I started, I did a few practice pieces on scraps of denim (legs cut off jeans, usually). I've also used some of Zoe's old tees. I figured if the applique sucked, I was only out my time and usually the tee looked better anyway. I experimented with different techniques on these practice pieces, too. For example, on this experiment, I tried doing a straight stitch as an outline to define the design even more. I liked the effect but decided it was way too much time to do it and my stitching wasn't as neat as I wanted, too.