Some tips:
General
*Playtest, playtest, playtest! I'm just saying playtest a level thoroughly, and definitely more than once. It's also good to have someone else playtest it.
*Save often! Save whenever you make changes (even the smallest), as you'll be thankful Contsruct2 or your computer crashes. Still doesn't hurt to save often, either.
*Remember to back up your projects daily. Not just copies on your computer, but you should also have offsite backups such as on dropbox.
*Be sure to test on multiple browsers (aside from mainly Chrome, IE and Firefox) and devices to ensure your game works well across platforms. Testing on mutliple devices is usually useful for games that support touch screen controls, although some games can be touchscreen or keyboard/mouse only.
Performance
*Use fps & " FPS (" & renderer & ")" via a set text to keep a measurement of your game's current performance, however make sure the condition is "Every tick".
*If your computer can support WebGL, it is recommended to enable it and/or switch to a browser that supports WebGL, and update your graphic card drivers.
*A good FPS to shoot for with designing mobile games is at least 30 FPS.
*There are two tweaks for performance when it comes to mobile devices. 1. Set "Pixel rounding" to "On" and 2. (if your game's background is obscured by objects like Tiled Backgrounds) Set "Clear background" to "No" and make sure every layer has "Transparent" set to yes.
*In general, common causes of poor performance are: Too many objects using physics, creating too many objects, using too many particles, using sprites instead of tiled backgrounds, using a slow or outdated browser, no hardware acceleration, and using too many loops. Also, remember: Always measure performance.
Layouts
*In general, I reccomend having three layers (from top to bottom): GUI, Objects, and Background. GUI would be for screen display objects (say, such as a HP bar), Objects would be platforms/the player, and Background would be for well...backgrounds. The background layer should also be locked, so you don't accidentally mess with it. The GUI layer should also have a parallex setting of 0, 0 while the background parallex should have a setting of 30, 30.
*When creating a new layout, be sure to copy the three layers you have made to the new one. Make sure their in the correct order, from top to bottom: GUI, Objects, Background.
*If you create a new layout and are copying gui sprites, make sure their in their proper place, on the gui layer and within the window outline.
Event Sheets
*Get used to creating groups such as controls, collisions, etc...not only is it more pleasing to eye, it's also slightly more organized than your events being all over the place. This also means bug finding is easier. Debug groups (for debugging purposes, such as warping keys or keys to dispense items) are also useful.
*You can hold Ctrl while clicking on a sprite to copy it as you drag your mouse away from the sprite. It's quicker than right click and copying but remember: These are instances, so they as treated as one group of something. So if you change a stat or behavior, the entire group changes
If you have any questions, feel free to ask!