The best way is going to be to get someone else to do it and if you dont want to ask someone close to you to help then go to someone who does it for a living. If going to a spa seems to expensive to you, check out some schools in your area. You can often get lower prices because they are learning or even a discount on that if you go at the right time. They can treat your back so that the blackheads you have go away and recommend aftercare so that they dont come back and you dont have to worry about this anyway.
As a little side note, some people like ';popping'; pimples and blackheads. Its kind of like a release for them and its fun. Maybe your girlfriend is this type of person. You never know until you ask. This is why some people become esthetians hehe.
Dont forget to ask the school if they need any models, because if you get a class that is practicing on males or for your specific issue you can get your treatment very discounted or even free. What's the best way to ';pop'; those hard to reach blackheads on my back w/o asking my girlfriend to do it?
Get your granny to do it.
My grandmother used to slaughter live pigeons and cook them. She also happens to be an estheticien. If you don't have a grandmother, use a friends' or go to an estheticien. A few treatments per year and her recommendation of a variety of pimple-poppin products will go a long way and will make sure your girlfriend stays in the picture.