Shampooing twice in one shower instead of shampooing more frequently (every day) could fix your dull, greasy hair problem.
Wet hair.
Put a R5 coin size of shampoo on your palm, spread it onto your hands
and distribute it evenly on the scalp only.
Scrub with your fingertips all over the scalp with enough pressure that you can actually feel your fingers massaging your scalp, for about 30 to 45 seconds.
You are now lifting the oils and buildup off your scalp.
Rinse well.
Repeat the process...
Scrub for a minute.
You are now removing the oils and buildup out your hair.
Rinse completely.
The soapy water that rinses out is enough to clean the ends of your hair.
Apply conditioner focussing on the ends only.
If your scalp is dry then use residual conditioner to moisturize and balance your scalp PH.
If your hair is long, now is a good time to comb out any knots.
Rinse thoroughly.
Wrap your hair gently into a towel.
Never ever rub your hair dry. It damages the cuticles and exacerbates dry damaged ends.
Leave the towel on for a few mins to absorb water.
Comb gently and style as usual.