Journals are places we record information to later re-examine patterns, and recall details. Journals are used in accounting and dream analysis for precisely this measure. If you are interested in growing and examining your sexual self, consider keeping a sex journal. This will be very personal, and so in the words of the great wizard Gandalf, "Keep it secret; keep it safe".
What to record
Each day record how many times, when and where you had sex or masturbated. The sexual self found in masturbation is often a completely different beast than others experience. Keep tabs on yourself too.
Be as detailed as possible when you record. Think about the sounds and smells, not just the mechanics. Record and rate positions you enjoy. Use stars or another symbol to represent how good each position was.
If you are artistic you may wish to include drawings or poetry. Write out your fictitious fantasies. Include pictures of individuals if possible (you can often find an avatar somewhere online). Maybe you will find out through comparison that your "type" is different than you thought.
When to record
The best time to record is right after an event. With sex and masturbation this isn't always possible (especially if it makes you sleepy). You may wish to keep your journal on an electronic device for ease of use. Most devices also have built in cameras for whatever you may need them for. Look at journaling or scrapbooking apps for good options.
When to reflect
Reflection is a personal choice. How often you reflect will likely change with time. Your personal activity frequencies will steer this process. Try looking back at the information after one week, after two weeks and after a month. If you have little data after these periods, ask yourself how you feel about that. Your own flow will come with time.
Make it a gift
If you are in a monogamous relationship, you could give your sex journal to your partner as a gift. Record detailed descriptions of the sex you shared, and what great things they did. If you want to tantalize your partner, include information about your masturbation.
Keeping a personal sex journal will allow you to remember more details (or names and faces) later. Recording the details of your sexuality will let you examine patterns of your own, and your partners', behaviours. If you are making a gift, consider who it is going to.
Will they be turned on by your masturbation, or just use it as ammo to harass you later?