The only way I have figured out how to do both is by dedicating each day to one or the other. For now: Monday and Wednesday: consulting. All day Tues, Thurs, Fri, Sat, Sunday: my startup. I don't even bother trying to work on my startup on Monday or Wednesday night. Any user requests from my clients are held until Monday and Wednesday. I even have them trained to email me: no more interruptions.
What kind of consulting work do you do? It sounds as if you can work from home? I usually work on site. There were some projects where I could work from home, and I liked it, but they tended to be the smaller ones that yielded less money.
I do contracting programming for local small businesses on their legacy systems. Has nothing to do with my startup, but is great experience anyway. Things come up that I could never envision, but I'll remember, so my software won't ever have these problems. I can dial up from home or work at their office. One client even gave me my own cubicle. I usually work at their site for several reasons: makes up for the loneliness of the other days, some projects have to be done on site (printers, phones, network, etc.), and, most of all, I want to see and hear their employees all day long. Can't get that from any book.
This way I get the best of both worlds. Try it.