I am terrible at spotting hypomanic episodes too. I went through an extended period where I'd be sleeping 2-3 hrs a day them working for 14 hrs without feeling exhausted ... Sadly when you're in the hyomanic moment, you don't think there is anything wrong with what you're doing. Mental illness is a cruel master right?!
I think journaling can help a lot and also it sounds so cliche but sticking to a high level routine will also allow you to see when you start to deviate. For example, maybe you could try giving yourself a weekly budget for online purchases and track them through your journaling. Amazon.com is my friend and my demon so I can sympathize.
Also, don't see the fact you're having a potential hypomanic episode as backsliding. It's part of the illness it's not a sign of weakness. How you deal with it is the key.