My dh is self-employed, and always has been.
Budgeting used to be hard until we figured out what we absolutely had to have each month. Then we subtract my income, and see what we need from him. And then we stash three months' worth of his salary in what Dave Ramsey calls a hills and valleys fund (just a separate bank account tied to our main checking). Example, with completely fake numbers:
Required monthly: $3,500
My monthly income: 1,500
Dh needs to bring in: 2,000
So we would keep $6,000 in our hills and valleys fund. Some months he makes much more than $2,000, and as long as we've got $6,000 in the H/V account, we put the rest towards paying off our mortgage or savings or whatever. Some months he may make $1,000, which means that we need to draw the remaining $1,000 from the H/V account.
This way we know with 100% accuracy how much we'll have to spend on regular budget items each month, and the only unknown is how much extra (if any) we'll have to throw at the mortgage, taxes, or savings/investing. In our experience, as long as you know that your regular monthly budget is covered, it's not stressful if the other stuff varies. We like having the three month buffer, because we never quite know when his income will spike again (record labels and show promoters all pay on different schedules).
So that's how we deal with that side of things. I had started to feel resentful of the stress that his varying income brought, because I'm the one who pays the monthly bills. This has worked out really well.
In terms of motivation, I don't know how he does it. He just loves to work. He does take about one week a year to lie on the sofa and re-read the entire Harry Potter series, and he does travel a lot too.
I'm hoping to work from home within the next few years, and the motivation piece is the biggest worry that I have.