I touch on this subject in the book I'm giving away. You can find it in another thread in this very forum.
The main idea is that, unless you are mentally prepared to move up and down, you shouldn't move up until you have a sufficient amount to endure a downswing and maintain your limits.
Here is a sobering statistic:
For the highest limit SNGs, a crushing ROI is in the area of 3-5%. Now, as you move down in limits your ROI will increase. I think at the lower stakes, a ROI of 10%+ is crushing for single table SNGs.
So, if at best you're looking at 10% ROI it makes sense that you're going to be missing the money quite often in single table SNGs or breaking even on large samples of games. You need much more than 5% buy-in rule to start to see long term profit in your games.
My suggestion would be to keep 50 buy-ins for each
limit until you get up to $22 and above. At that point start to keep even more buy-ins because as you move up, your ROI will drop.
So for $5 SNGs, I would want to have $250 plus an amount I could be comfortable with if I had to move back down to $2 (like $50 to $75). Meaning, I wouldn't move up to $5 games until I had around $300.
This may take a while to achieve, but what it will do is ensure that, if you are a winning player, you will only have to move up one time instead of having to bounce back and forth (which can actually take much longer than what I am suggesting).
While not all bankroll strategies are set in stone, this should provide some direction for you. If you are able to easily replenish your bankroll, you can take more risks as well.
Anyway, hope that helped. Here is the link to the other thread: