All my life I've had oily hair. About 24 hours after I wash it it's greasy and gross again so I usually wash it every night. However there are days when I get home too late or I'm just too tired to wash it so I'll put it off for a day. You'd think with all the grease I wouldn't have a problem with flakes but it actually creates them because the extra "moisture" on my scalp makes that skin come off and I end up with white bits in my hair. The extra oil makes my scalp itchy too so scratching it makes even more flakes.
There are plenty of products out there that are marketed towards this problem and its hard to know whats going to work for you, but there is something so simple that my mom's been using on me since I was a kid that I always come back to: Apple Cider Vinegar.
Yup, the smelly brown stuff that anyone with a nose should fear.
When ever I start getting a flaky or itchy scalp I get a big cup and fill it with 1/3 Vinegar and 2/3 warm water. I put it on in the shower and let it sit while I wash my face. Then I rinse it out and use my usual shampoo and conditioner. This totally cures the itching and stops the flakes. It does leave your hair smelling for a while so don't do this if you're going out in a few hours.
I believe you can get this at most grocery stores. Be sure to use a organic/natural brand like Bragg's as others can be dilluted and have added chemicals which may make them less effective. You want the purest form you can get.
Apple Cider Vinegar is also good for curing ring worms (soak a cotton ball and tape it on), relieving a sore throat (gargle & spit, don't swallow), and many other things (my mom uses it to clean our bathroom and kitchen floors).