How Do You Teach Yourself To Just Say No?

As usual this weekend I had more things to do than I had time . Besides being A full time student I have A small cake business called Camela's Classic Cakes. I Try to limit myself to just one order per weekend. This may seem like an insignificant thing. But the cakes I make are just like the ones you see on the TV shows . Yes really , just like Ace of cakes or Cake Boss. The reality is they don't show you on the TV shows all the prep work one of the other employees did . All you see is the decorators creating a beautiful work of art. Well there's just me and making cakes is extremely time and labor intensive. But I digress , the topic is how do you teach yourself to just say no? So I had a baby shower cake order for this weekend and as it turns out it was also one of my best friends fortieth birthday. Even though, no one ask me to make her a cake I couldn't stop myself. Some how I found myself compelled to call the woman in charge of the party and volunteer to make the birthday cake. A four tier cake that looks like a stack of presents. Some where into the third day of working 12 hours per day on these cakes I came to the realisation that I am getting too old for this $#%*. This isn't the first time I have gotten myself in over my head and it probably wont be the last . And it's not that I wouldn't do anything for my friends or family. I don't regret making my friend happy. She was completely surprised and loved the cake. It's important to acknowledge that friends are valued and loved especially at the mile stones of their lives. I think that I have come to one of those mile stones , the one were you have to acknowledge your physical limitations . I'm in my mid forties and let me tell you getting old ain't for sissies. So how do you go about training yourself to just say no? Any one with any ideas please respond. Anyway I pulled a muscle in my back rolling out the fondant icing. So I managed to stay at the birthday party just long enough to help serve the cake.