Box cake + sagging = worst baker ever

Dear Tricia, I am a really big fan of cheap box cakes. Especially the one that has the different colored rainbow sprinkles. But every time I make one the middle sinks in. Should I be embarrassed about my sagging problem? Please help.

Yes. You are officially the worst baker ever. What's worse than you being the worst baker ever is that you like cheap box cakes. Just fork over the extra 79 cents and buy the Duncan Hines brand already. You're embarrassing yourself.
That said, I can help you with your sprinkle cake conundrum. First of all, are you measuring everything correctly? I know its tempting to be all chef-like when you're baking and just throw in a pinch of this and estimate what a cup of that looks like, but with baking, you really do have to be exact if you're looking for the perfect sprinkle cake.
Do you live at high altitude? If you do, don't add as much water. A liquidy cake batter won't want to rise and stay risen. (See my pregnant cake post for an example of what proper cake results will look like)
Is your oven pre-heated? While a hot oven will force a cake into its submission and make it rise like no one's business, an oven that's not done pre-heating will basically just sip a cocktail, nudge your cake and ask, "Do you want to bake or what? Yeah, I didn't think so."
Make sure your pan isn't too big. It will definitely sag if the pan is too wide. Follow the directions on the back of the box and use their recommended cake pan size. My guess is that this is probably your problem: seeing as how you won't even buy the store brand cake mix, you probably don't want to fork over the dough for the right cake pan. Hey, I understand: times are tough. (cheapskate)
Last, when you are mixing the batter, bake it right away. Cake batter doesn't like to sit without being baked... it's on a mission to fill your belly. Let it do its job.
Just to be sure I didn't leave anything out, I checked the Duncan Hines website for cake baking tips, but after I read part that said "don't frost a warm cake" I knew they weren't going to be much help. And they weren't. They basically said half of what I said here but without the irreverent, judgmental humor. Where's the fun in that?
*UPDATE*
I forgot one of the most important baking rules ever!! Don't slam the oven door. When you're starting to get curious about how your cake is doing and you open up the oven door to check on it, close the door very gently. Shaking the cake before its completely baked just ruins the structure and collapsing is inevitable. Or, just look through the oven window instead of opening the door. Resist the temptation. Resist. The. Temptation.