Personally I use a small spray-tube that was originally a sour candy spray. After a good wash you're left with a great sized mini spray that you can take around with you in your bag or purse etc. To my water I add 4-6 drops of whichever essential oil I've opted for. Personally I'm partial to lavender. Then I can spritz away and add a little scent to my other wise scentless dreads.
You can use whichever essential oil you like and little vials of essential oils can be found at your local health food store or some pharmacies. As a quick tip - Rosemary is good for dandruff, Tea Tree Oil is an antibacterial and antifungal and Peppermint can help if you need a little cooling behind the ears / neck.