Continue to work while you write.
When writing your answer, start working on the next part.
As an example, If I'm calculating 4/68, while writing how many times 4 goes into 6 (part one), I'm already thinking about how many times it goes into 28 (that's the next part).
Work with random numbers.
You're driving in your car on route 82. Why not do as many calculations as you can with those numbers? Right now, I can think of 8+2, 8∙2, 8/2, 2/8, 8-2, 8^2, and 2^8. Though the last one might be a bit hard for now, you get the point.
Practice.
The bad news: This part is boring. The good news: You only need to practice for a minute a day. Personally, I use Brain Age2 for this. Anyway, all you need to do is use a random number generator, and solve the problems it gives you. Don't have an RNG? Make one. (Steps at the end of my list)
Put down your calculator.
This is a given. Obviously, it will help. Never use a calculator unless there is a time limit.
Memorize more times tables.
In school I learned the multiplication tables from 1∙1 to 12∙12. As a personal recommendation, I suggest learning from 1∙1 to 20∙20
Use familiar numbers
What is 14∙7? Give up? Try 7∙7, then multiply that by 2. You can use this trick on many numbers.
How to make a random number generator (This is easy):
- Take a piece of paper and cut it into 9 pieces
- Write the numbers 1 through 9 on those pieces of paper
- Put them in a box, bag, etc
- Do steps 1, 2 & 3 again
- Cut a piece of paper into 4 pieces
- Put +, -, X, and / on them
- Put those in a box, bag, etc
- Now, pull out one piece of paper from each container, do the calculation, and put them back in
good luck!