There are many ways to increase the chances that you will retain what you learn. I list here a few that I know:
The more often you review what you learn, the more likely you will retain it even longer. By review, I mean stopping to look back over what you have learned to remind yourself of the major ideas and how they are connected. You may want to skim over what you've done, or think about it, or say it to yourself, or write a short summary, or draw a picture.
People recommended to me when I was a student that you should review what you learned at the end of every study session, at the end of each day, at the end of each week, and at the end of each month.
So, when you have been studying for an hour and you want to stop, before you leave, try to summarise what you learned. At the end of each day, plan a short study session where you look back over the whole day and summarise what you learned. At the end of each week, plan a study session where you look back over the whole week and summarise what you learned. Similarly at the end of each month.
Personally, when I was a student I used to use my daily train journey home to quickly jot down in a diary what I learned today. On the weekend, I would have a session where I looked back through the week's diary and thought about the week as a whole.
In your particular situation this has an advantage that if your study is interrupted at any moment, then you have been reviewing as you go anyway, so you are more likely to retain across a gap in time.
Explicitly make connections
You are more likely to be able to recall things you learned if they are connected to other things in your mind already. Imagine your mind as a map with towns on it an roads between them. If you want to get to town A, the more roads there are between any town and any other town, the more chance you have of hitting the right road to town A eventually. But if town A is only connected to one other town, you have much less chance.
Also, the sensation of understanding happens when new ideas are connected to existing ideas in your mind (literally the brain cells connecting to each other).
So you should explicitly try to make connections. Constantly ask yourself "How is this similar to other things I have learned?", "How is this different to other things that look similar?", "Where does this idea come up?". Another strategy is to draw the connections on paper, in a diagram like a mind-map. Don't just try to connect the ideas within one topic, but try to connect to other topics too. A memorable example of this for myself was when I explicitly made connections between algorithms in polynomial arithmetic to algorithms in whole-number arithmetic.
The above paragraph would seem to suggest that the only connections are to other ideas in maths, but the more connections there are to anything the better chance you will have of understanding and retaining. So ask yourself if the things you are learning remind you of other tasks you know how to perform, or life events you've been part of, or movies you once watched, or people you know, or foods you ate etc. Personally for me the movies I have watched often give me great connections to draw upon.
Use multiple senses
The connections mentioned earlier are not just to ideas, but also to sensory input. Each of your senses is stored in different places in your brain and is associated in your memory with different experiences and ideas. So a way of increasing learning retention is to use more than one sense.
When you are summarising the day, write a list, and also say your list aloud. Construct models out of play-dough or paper. Associate a sound-effect to a particular numerical operation. Write different types of ideas in different colours. Draw pictures with your finger on the back of your hand so you can feel them. When you explain an idea to yourself, use hand-gestures as well as words.
Hand gestures are particularly important -- there is research to suggest that the movement of your body and in particular your hands plays an important role in all learning.
Practice using the ideas and choosing which one to apply
As others already mentioned, using the ideas to do something will help you retain them. So seek out problems to do. The best problems are the ones that involve more than one idea, or involve a bit of playing around to get somewhere. These ones will force you to make more connections and really use what you have learned to do something new. Puzzles are another good thing to do because they often use ideas in unexpected ways. Don't forget to review the things you learned while doing problems at the end of a problem-solving session!
While it is obviously good to practice each separate skill or type of problem separately, it is also important to have practice at choosing what skill to use or what type of problem the problem is. Many excercises are in blocks where all the problems are about the same thing or require the same skill (such as those on Khan academy). You also need to seek out exercises which mix together different types of problems and skills. Choosing one from each block in a textbook is a good start.