First of all, I'm not quite sure what you mean by "commit a sin" while praying the rosary. If you mean, have a distracting thought, well, that's not a sin. Just collect yourself and bring your attention back to the prayers. Here is a good strategy that I use--if my distracting thought is about a person, I catch myself, and offer the next Ave Maria for that person. If you mean blasphemous or sinful thoughts, that's probably just some demon trying to get you to give up praying. Ignore him and keep talking to Jesus and Mary. They HATE those names you're pronouncing, and when they see that you're not going to give up, they'll leave you alone.
I would not set fences around your rosary. It's simply good to say the rosary. If you say five decades together, great! If you can say all 15, go for it! If you say one decade at a time throughout the day, that's great, too. Just try to get 5 in every day. I, personally, have to do 5 all together (especially in church or with family to get the plenary indulgence), but Abp. Sheen tells of a busy doctor who would say it through the course of the day. In times of temptation or suffering, I have turned quickly to the rosary and said a decade. The thing is, if you start feeling guilty, discouraged, or scrupulous over some detail, or for not saying 13 million full rosaries every day, then tell the devil to go to hell, because he's tempting you with these scrupulous thoughts.
If you forget the Our Father, you could just say it then and continue on, or repeat the decade like stjude suggested, knowing that, whatever you decide, it's not a sin. If you lose count, don't get hung up on it. Don't be too scrupulous about it. It's more important to pray it from the heart than to mind all the rules.
Did you know that throughout Latinamerica, the only introduction to the rosary is the Act of Contrition? Their rosaries have only 1 bead between the crucifix and the first Pater. Did you know that the O My Jesus prayer was only added in 1913, and not even part of the rosary proper? There are any number of aspirations you could add after the Gloria, or none at all. (Personally, I like to do what Our Lady asked at Fatima.) My point is, although the rosary is said in a certain standard way in groups, you don't have to follow rigid, strict rules when you pray it alone. Just remember who you're talking to.
And also, you should know that the rosary is this:
1 pater, 10 aves, 1 gloria--for each of 15 mysteries. That's it. Everything else is optional and is a result of customs or personal devotions.
You might read these suggestions:http://www.ageofmary.com/holyrosary/helpful-suggestions/
Also, check out St. Louis' 2nd method:http://www.ageofmary.com/holyrosary/montfort-method-2/