I have some myself, hopefully there are some people that do not know about these:
1. Doodads that cover the ground like a texture, such as tarmac and decals, can be made more transparent by lowering their height. To do so, double click the doodad and set the height to for example -3.0. The lower it is, the more it fades out. This is useful whenever you want your doodad to draw less attention and blend in with the ground textures.
2. I struggled a lot with cliff doodads before I realized that the method of lowering their height is probably what is intended. You just simply place them near the cliff, at their right angle, and then lower their height in the same manner as above, until they are placed where you want.
1. You can exchange specific textures in your selected Tileset with textures from another Tileset. This can create unique terrain effects, as long as you don't mash textures that look terrible together... The way to do this is open Map Textures (I think) from the menu bar, and click edit textures. Then in the data fields you can select what textures to swap.
2. Some times it is really hard to edit textures with the normal camera view. The camera point of view can be changed using the menu bar, but I have always found that to be quite clunky. Instead, I usually go to Points, select/create a starting location and right click it - select View Point. Then you get a completely 90* view of your map, and you can go back to Terrain editing easier. You can change back to normal camera view from the menu bar. There may be a faster way to change camera that I do not know of, though I have had no problems with my method.
3. I think most people know this, but: When adding water, you need to change the height of your water for it to function as intended. The way to do that is click 'Edit Water' (I think) when you are in the water tab while Terrain editing and have your specific type of water selected. Then you lower the Height field until you found your desired effect (test document to see). If I remember correctly, I think you generally want it to be lower than 1.0, otherwise it will just flood above regular cliff heights.
1. If you have a map with loads of triggers, and get any kind of error when you test it in-game, and you have no idea what trigger caused the error, the process of elimination can be very helpful. Right click a trigger and disable it, test document to see if it still produces the error, then go on to the next trigger, etc. When you find that suddenly the game does not produce an error, check the trigger you last disabled. Obviously, this does not work in all circumstances.
2. When it comes to entering values, see what type of value is being 'asked for'. For example, text messages may be displayed to Player Groups. A player group is not the same as a single player. That means you probably need to create a player group for that player. Similarly, another important thing to keep in mind is the distinction between integers and reals. Specific players come in the form of integers 0-15. I have many times been confused, not finding the options I wanted, then realized that the trigger required a different form of value input.
1. If you are not sure of what you are doing, be a little careful with messing up too much in the data editor if you have spent a lot of time on your current map. While it is very powerful, it can also be somewhat user unfriendly. However, there is a way to reset specific objects in the Data editor, so that it goes back to the original form. To do so, right click an item and select 'Reset to parent' (if I remember correctly, it is something like that at least).