There are a couple of areas I actively try and foster in order to improve my "intelligence".
CreativityThis is probably the most difficult one to improve and is seems to be directly related to Knowledge and Memory but there are a few things that I have learned I can do to improve my own creativity.
Experience new ideas or new knowledge is a big one. When something new enters my brain and I begin to amalgamate it with the rest of my knowledge new and interesting connections and ideas pop up. This however is not really related to new experiences. New experience tend to make people cultured and have new points of view but traveling and visiting new places or meeting new people has never really enhanced my creativity in any way. Every once in a while I might see a painting that can give me interesting ideas but this is so few that to me it is not a productive method.
Take time to think of new things. I always can tell when I am stuck in a rut it probably means I am not taking enough time to think. I have set a reminder in my calendar on my computer to take at least 5 minutes everyday and drop everything I am doing and spend that 5 minutes only trying to think of new and interesting things. It is amazing how much this works.
Creativity seems to feed off of creativity. Talking to other creative people and seeing other creative projects or ideas feeds me. Some of my best ideas happen after sitting through a bunch of interesting scientific talks in a day.
Streamline your life. When I don't need to think about problems or dinner or food or money or whatever I can spend all my time thinking of interesting problems. I like familiarity. Do you think about how to put on your clothes? or think while eating? Probably not. Once this autonomous system takes over the mind can live outside the body. The more things in my life that are familiar and similar the easier it is to spend lots of time thinking and attempting to solve problems or come up with creative ideas. I can't stress how important this aspect is. People might think I am weird because I want to use the same spoon, cup, bowl, sit in the same seat and walk the same path. But what they don't know is that this familiarity allows me to do everything autonomously and I can spend that time thinking.
KnowledgeI think so many people give up on knowledge at an early time in their life. They figure that they learn enough with the job or that they know enough so they don't need to learn. Knowledge, to me, is intimately linked with creativity.
Learn. I spend so much of my time learning. When I am walking alone I try and listen/watch lectures or podcasts. I constantly try and teach myself new subjects. This takes alot of dedication but it allows me to have an idea and then apply the idea and make it real. Ideas stay ideas too much because people don't know how to apply them or are not willing to learn enough to apply them.
Do. Doing things is what builds those conceptual connections in my brain. When I am programming or building electronics or doing a science experiment I build the conceptual connections of things I only understood in passing. Someone can tell me how NMR works but if they can't sit down and run a spectra or work with a spectrometer then the knowledge is only theoretically useful, which is ok in some cases but for me it is less useful in most cases.
MemoryI think this is the most difficult to improve. Some people appear to have an inclination to memorize things easier maybe through their upbringing or practice but memory really takes practice.
Practice is really important. I practice memorizing things. I train my memory. Learning how to use ones memory efficiently and beneficially is what makes someone better at "memorizing" than other people. A really good book on the subject is Moonwalking with Einstein by Josh Foer. The book is about training to compete in the World Memory Championships and how memory is more of a learned skill than it is an inherent talent. I wrote a simple Perl script that I run through a couple times each morning, it gives me a number of random digits of my choosing and 15 seconds to memorize them and then it asks me for them. This works pretty much only my short-term memory but I have seen the benefits in my long-term memory. I am up to 13 numbers in 15 seconds and have been stuck on 14 for a bit now. My goal is 15 number in 15 seconds. I also have this neat app on my Android phone Memory Trainer it has a bunch of different "games" that train image recognition, spatial memory and a bunch of stuff. It is pretty cool. I never however understood the N-back game I think it doesn't work correctly. Also, it is not that difficult of a program. After a few months using it one can complete the highest level as a "Memory Master" but you can go back and choose targetted workouts on the games that give you the hardest time. Really nice way to "waste" time besides something like Angry Birds(do people still play that?).
Experience is very important in memory. What does the number "8" feel like to you? To me 8 is a good number it is my birthdate (Feb. 8th) and also is related to my birthname (Josiah became King when he was 8 years old). I can feel the number 8. I also like powers of 2 because they are based on the binary digit system, like 256, 512, 1024, 2048, &c. The more you experience numbers or objects the more you are familiar with it and the easier it is to remember. Experience also allows one to learn what works best for them. Maybe associating smells or people or shapes works best for you. Try it out and see what happens.
I really wish I knew about metacongnition earlier in life or that I was taught how to think in school I am sure I would be much better at processing and synthesizing information.