How do you guys do this? When you have enough buyins to next level do you then take one match from that level and if you lose it go back to lower level or what?
Are we speaking HU hyper sng or HU turbo sngs? For hypers, I usually go 100 BI up until 7$, 200BI at about 15$, 300 BI at midstakes. These are all numbers for HU hyper sngs. I think that you can certainly go with less, but 40-50 BI swing happens from time to time and it can affect your mental game heavily. Imagine you have just 150 BI and you lose 1/3 of your roll for 2 weeks, when you had to work for it for a few months. It also depends on the rake/rakeback deal you are getting. My examples are for pokerstars.com.
For dropping limits, I think you can go a bit aggressive with your bankroll and try f.e. the 15s when you have 100 BIs and drop back to the 7s when you've lost 30BI. Once you regain-you try again.
I am not so sure how it works with the turbos, but I would imagine it's gonna be a bit easier on the swings, so lower BR will be needed.