I have a character that I want to develop slowly through the campaign. By this I mean I want him to slowly mature and change his personality through the sessions. What tool or trick can I use to keep myself on track when roleplaying him?
This answer comes from a MMORPG-oriented roleplaying group, but it's a good one!
There's this technique called a character diamond. Basically, you choose four words that define the personality of your character: for example, my fighter Collin is modest, taciturn, gullible, and underwhelming. You're looking for things that don't overlap; the link provides a longer explanation, but that's the core of it.
That gives you the heart of the character. You can actually write them down for reference, which is maybe a bit goofy but I find it can help. Then, when you're thinking about more transient or temporary traits, you can think about how they relate to those central concepts. You also know that if you want to change one of them, it's a big deal and deserves a lot of preparation. Or maybe just a real life-changing event, but either way it'll remind you of the stuff that was meant to change slowly.
One thing you could do, possibly in tandem with Bryant's response above, is use your character's traits in a way similar to "Keys" in a game called The Shadow of Yesterday. In that game, every time your character does something directly related to a Key they gain experience. So, if you had a Key called "Greed", you might get experience whenever you hide loot from your companions, or put yourself in incredible danger in order to gain something of monetary value.
Instead, you could decide that after your character exhibits or acts upon a trait a certain number of times that you will transition his behavior in that regard towards something different. You can just put a series of check marks by the trait to keep track for yourself.
The Burning Wheel RPG uses techniques specifically for this kind of development, and it's reward cycle is based heavily on this.
At the core of the system lie Beliefs, Intent and Traits. Beliefs tell you what your character wants (more long-term), and traits are personification(s) of Beliefs. (I'm simplifying).
Check it out if you like.
Ask for feedback on your interpretation to other players. Ask them to keep and eye on those special details that make your character special, and try to think in advance how and where are you going with them.
I find usefull to mimic some expresions, habits or ways from films or books, adapt them to my character, and then develop them in some way or another with help from the group.
I generally start with a rather empty shell (but still with a proper backstory), and then let the campaign define his/her morale, objectives, hopes and dreams. Sometimes, there are critical events that form a character, and they are decided by a die throw. I save down these critical events in her character sheet, and let my fantasy fill the gaps on how the character behavior evolves.
For example, a character of mine, Female elf, cleric of Lurue, had to save his comrades who kept falling down a cliff. She was determined and helped people despite her poor skills in climbing and high danger to fall to her death (we are talking of a 1st level character against a CD 18 cliff).
Later on, we had to go into an academy and study sorcery. Due to very poor throws, she was unable to learn anything, and a critical 1 lead her to throw the book out of the window.
These events defined her as a very good person, but rather unable to focus, and not particularly skilled with studying. As a result, I left her reach the 17th level Cleric with just one grade in Knowledge (Religion).
She married and had a child from a paladin (another player's character), but then was abandoned by him. She kept her wedding dress "as a memento for regrets".
Later on, I reused the character as a young apprentice in a prequel campaign DM'ed by me. She kept the same characteristics: a very caring, generous and good person unable to deal with formal training, and respect for the rules. She is a character which all my friends and I remember with great affection.
There are quiet a few good examples of techniques and tricks in literature -- in fact, it could be argued that any good literature will contain just that ^_~. Pick books, TV series, and plays to read. Most of them will have character progressions. Just as an example, I would suggest looking at Babylon 5, in particular G'Kar and Lando Molari as examples. Both change drastically through out of the show in response to events that are happening. Many of those events are choices those side effects full y escape them.
Define critical moments that changed your character's outlook. Make sure you refer to those from time to time -- "Remember that time in X" --. If elements of that are reflected elsewhere, you can assume a flash back: Your life changed in a bar while drinking whisky. Every time whisky is offered, make sure your character reacts strangely.