Some tips from my side, some of which are already mentioned:
- Always make sure all the ingredients are at room temperature before you start.
- ALways use fresh ingredients, like Mehnaz said. No 2 year old baking powder is to be used. I have seen it first hand how it can make or break your cake (no pun intended :D)
- Always follow the recipe exactly. There are different ways of mixing different types of cake. If a recipe states to only mix until just combined, use a spatula to do the job. If the recipe wants you to beat for a good 2 or 3 mins, do just that. For some types of cakes, overmixing can result in a tough, hard cake.
- Double check your oven temperature. Like Mehnaz said, either invest in a oven thermometer or if you know your oven runs hotter than you think, adjust either the baking temperature or the time accordingly. For example, i have an electric oven and i know to check 5 mins before the baking time suggested in the recipe since my oven is hotter than i think it is. For cookies, i check about 2-3 mins before.
- Baking is a science. Its not like cooking. You cannot substitute oil for butter for example, and expect the results you want. Different ingredients will yield different results. But for somethings its ok, so if you dont have vanilla essence, either omit it or use another flavor. Soemtimes if a recipe calls for lemon zest and i dont have that, i will use lemon extract with good results.
- Like sumla mentioned, make sure you are using the right sized pan.
- Always preheat the oven appropriately before baking.
- Dont keep opening the oven door to check on the cake. I usually dont check the cake until i think its almost done since my oven runs hotter.
- Overbaking a cake will result in a tough cake like sumla mentioned. you should only bake till the sides have come away from the pan and a toothpick or knife inserted in teh middle of the cake comes out clean.