I'm definitely not a pro when it comes to baking, but I've been doing it since I was a kid (thanks Mum for letting us mess up your kitchen - even though we were always supposed to clean up I bet we didn't!) and I've picked up a few tricks along the way - things I don't even think about when I'm making something, but which can make the difference between something average and something awesome. I thought I'd share some of these with you guys - and please add your own in the comments - I'll bet you can teach me as much if not more than I can teach you!
If you are just combining ingredients, try not to stir more than you have to. Once you've added flour to a recipe the more you stir the more the gluten will develop - which means cakes can end up chewy and dense if you stir too much. Likewise, if you've beaten air into an early stage of a recipe (e.g. egg whites or creamed butter and sugar), each turn of your spoon will remove some of the air, and air acts as a raising agent so you want it in there! Recipes should tell you if more than just combining ingredients is required (and in some cases this doesn't matter so much) - but otherwise try to minimise your mixing.
I think this is probably the easiest mistake to make with baking - naturally people are afraid of not cooking their treats enough, but though your structure might not be great if a cake or cookies are undercooked, they'll usually taste better than if it goes in the other direction! Remember that anything that holds heat will continue to cook for a bit even after you take it out of the oven. Mostly I use springiness as a gauge for cakes - press lightly on top and when cooked it should spring back - rather than sticking a skewer in. If I do a skewer test (mud cakes, for example, will never really get springy) I'm not looking for a clean skewer, but cooked crumbs. If the skewer comes out clean it oftentimes means the cake is dry - overcooked. If you do overcook a cake drench it with a syrup before you ice it - that will often bring it back. :-)
Use good ingredients
I'm not a disciple of the every-ingredient-must-be-of-the-highest-quality-possible school of thought - use common sense to determine where it matters. Chocolate is where I notice this most; I never use the stuff from the baking aisle (though I haven't made anything which needs to set hard since I became such a snob and haven't yet mastered tempering so there might be an exemption clause on that). Usually I go with either Whittakers or Cadbury; if the recipe is really putting the chocolate on display (e.g. chocolate mousse) I'll use Lindt or Callebaut.
Put a little extra effort into presentation
It doesn't have to take hours to make something look amazing - but piping icing instead of spreading it on (easy to self-teach - you just need a piping bag and tips) or trimming the top off your cake so that it sits flat (bonus: you can nibble on the offcuts!) can give that little bit of wow factor - the visual experience is completely intertwined with tasting, so something that looks good is far more likely to be enjoyed. It's true that I've spent hours fiddling over fussy cakes - but that's definitely not my norm!
Pretend it was supposed to be like that!
I'm actually no good at pretending - I over-analyse everything so even if something comes out well in the end I'll rabbit on for ages about how it was supposed to be and what I should try next time to fix it - but icing sugar can hide a multitude of sins; cake can be converted into cake trifle, and failed just-about-anything can be made into truffle-type balls, sometimes just by rolling into balls, sometimes with the addition of cream cheese or some other icing-type substance, or by coating in chocolate or cocoa or coconut. Same principle applies for slice - the key is confidence. ;-) No-one will ever know!
So what can you add into this list?