If you've ruled out pre-seminal discharge (precum) and STD's, you might consider NSU, as was said earlier by gymandee, and you might also consider a fungal infection, too. Applying a cream to treat jock itch after a shower will be beneficial if this is the case. I'm assuming here that the discharge isn't coming from your urethra, and that you've got no open sores. Showering with a mild soap morning and night, and keeping your goods dry and clean will help.
If you're very sexually active, or masturbate frequently, you should try to take a few days off to let any skin irritation heal up. I get a little raw myself if I have twice-or-more daily sex sessions for more than a couple of days. If you're getting busy every day, or having sex and masturbating in the same day, you might be putting microtears in the skin of your penis, which would leak plasma serum- slow down for a week or so if this is the case, and let your penis rest!
Non-specific urethritis (NSU) is a common cause of discharge. ‘Urethritis’ means inflammation of the tube, and ‘non-specific’ means that is difficult to know the exact cause. NSU causes a discharge that is usually clear, and is worse in the mornings. You may find it uncomfortable to pass urine, and you may feel irritation along the urethra inside the penis.
NSU is caught during sex. Several types of bacteria may be responsible; about half of cases are caused by Chlamydia and about 40% are caused by other bacteria (such as Ureaplasma and Mycoplasma). These bacteria do not cause a discharge in women, or any other symptoms in the early stages, so most women do not know that they have an infection.
I suggest seeing your Urologist