I wash my hair every single morning and have done since I was 13. Until I started my current regime a couple of years ago, I had to wash it again to go out in the evening as it was greasy again by late afternoon (I used to keep shampoo and a hair dryer in the drawer of my desk if I was going out straight from work). Dry shampoo just made my hair look slightly less lanky, but dull.
Friends were constantly giving me 'helpful' advice to 'train' my hair to need washing less offen, but I knew that mine wasn't just normal oily hair and no amount of 'training' it would make a difference. It always looked great after it was washed in the morning, but was already starting to look greasy again by 4pm in the afternoon.
In desperation I went to see a dermatologist who also speciallises in trichology. He explained that people who don't have overactive sebaceous glands on their scalp (the cause of excessively oily hair) don't understand that leaving it for longer between washes does no good at all and is actually detrimental to scalp and hair health. So I had been right to ignore my friends' well-meaning advice. The fungus that causes dandruff and seborrheic dermatitis feeds on sebum (the excess oil) and multiplies very quickly. Thie fungus causes scalp irritation, leading to yet more sebum production, and weakens the hair root. This weakness is exacerbated by the excess sebum blocking the hair follicles and affecting the flow of blood and nutrients to the hair root. So it's vitally important to keep the scalp clean and oil free, not just because greasy hair looks and feels so horrible, but because the excess oil affects the health of the scalp and hair follicles.
Anyway, I can tell you what works for me and I hope you find it helpful. It's been life-changing for me as my hair used to constantly preoccupy me and affect my self-confidence.
Every morning I alternate between Seboradin and Nizoral shampoo, massaged right into my scalp and left for 3-5 minutes before rinsing. I follow this with a normal silicone free shampoo (choose clear rather than creamy shampoos to avoid silicones) and rinse it straight off. I only use conditioner on the last couple of inches of my hair to make sure it doesn't end up anywhere near my scalp. For the last two years, I've also taken 2.5mg oral minoxidil every day which strengthens the roots of my hair and keeps it in the anagen (growing) phase for much longer. My hair now doesn't shed in tbe hairbrush or shower, so it's very noticeably thicker than it used to be, and even by the next morning it doesn't have that 'stringy' look anymore (although I still wash it anyway).
Understanding the 'science' behind my greasy hair has been so beneficial and I now treat it as a medical condition that can be successful managed.