Heya
Well Thats no problem, in Photoshop. Never draw on the background layer, Start a new layer and sketch out or create a silluette of what you are planning to draw. I use silluettes as it leaves you more creativity in the form you make and isnt as restricting as a line-drawing of a shape/figure.
Then when you start the actual drawing, create a new layer ontop of your sketch/silluette and draw on that. To make things easier, try making your sketch layer 50% opacity or even less so you can only just see it as a reference.
Thats one method
As for things i wish i'd known...
1.
Shadows = More saturated and Darker.
Highlights = Less saturated and lighter
Seems simple but its bloody hard to get right!
2. Draw in Black and white to get your values correct. Then add color after.
3. Whenever possible use larger brushes when you plan on having texture on something you're drawing. Small brushes leave heaps of lines and make your drawing look unrealistic.
Thats all for now i gotta run
haha Talk soon!