Step1
Realize that if you are 60, and your bones ache, your 85-year-old parent probably hurts more. If your mobility is limited, theirs is probably more limited. And, if you resent what age is doing to you, they probably feel more strongly about it. Allow them the freedom to make as many of their own decisions as possible for as long as possible, even if it's just what vegetable to have with dinner.
Step2
Living with an aging parent is similar in many ways to raising a child, but in reverse. Your parent is used to being in charge, handling his own finances and just generally being in charge of his own life. The typical aging parent has some days that are better than others. Parents will have more good days if they are encouraged to stretch themselves on those days. They should drive for as long as they are safe and comfortable doing so. They may move slowly, but that walk they want to take around the block is hugely important for their physical and mental well-being.
Step3
Know when you need to step in, but do it as tactfully as possible. If Mom can't open her bottle of water, don't just grab it and do it for her. Ask if she could use some help. It wouldn't hurt if you point out that sometimes you have a hard time, too. They need not to feel like a burden, and to know that they aren't alone in what is happening to them. We ALL grow older every day.
Step4
Anticipate their needs, but again, do it tactfully. Check on Dad during the night. Make sure he has what he needs easily available. Make sure your parents are eating properly. Left to their own devices, they'll probably eat what they like that's easy. If that's a problem, check what they've had, and make a habit of making two sandwiches at lunch, one for you and one to offer them. They may turn it down because they're full of crackers and coffee, but eventually they'll eat it.
Step5
Go along with them to their medical appointments. You want to hear firsthand what is going on with them. Insist that the doctor treat them with respect, or find another doctor. With you there, many medical professionals will be tempted to talk only to you, as if your parent isn't capable of understanding what's going on. That may be true to some degree, but your parent needs a doctor who sees each patient as a person worth individual attention and respect.
Step6
Take this time to find out the things that you won't have a chance to find out after they're gone. Filling out family trees, hearing the stories of their childhood, all the things they know are important. They may make you crazy chattering about all of it, but one day they won't be there to do it, and you may feel differently.