the main pointers i can give are (based on techniques i use most)
-download as many brush packs as you can. it's always handy to have a few good brushes to give some variety to your sig.
- always have some good c4d renders at hand to give your backgrounds a nice abstract touch
- always use clean renders. and use some that have features that can be accented. i usually base the majority of the sig around the render.
- pick colours that work with each other. here's a colour wheel to help
- don't be afraid to experiment with effects, smudging and blending options. if something goes wrong it's easy enough to undo or delete the layer.
- picking the right font is crucial to any sig. also positioning of the text is important.
- by using white and black with a soft brush you can create good lighting effects to add an element of depth to you render
- use clipping to your advantage. to do this simply select a layer, place it above the layer you want to clip it to and alt-click on that layer. when a layer is clipped to another only parts with the second layer underneath will be visible.
- don't just leave the focal point of your sig on top. make sure to use things like clouds and brushes to blend it with the rest of the sig.
- don't bother trying to get the exact balance of colours right from the beginning. just wait until everything is as you like it and use gradient maps which i have highlighted the icon for here
just click that icon and select gradient map. this tool won't make awful colour combinations look good but it can be used to add depth to your chosen colour scheme.
-make sure your sig is a good size. too big and you'll have too much space to fill. too small and you won't be able to fit much in.
hope all those points can give you a general idea of what to do in a sig
here's a sig tutorial i found really helpful and where a lot of points i mention here a described in more detail