Just a few timesaving tips off the top of my head:
Type C codes - only work out the number of letters needed (usually the first three).
Types P (number sequences) and L (codes) - if you can't work it out, it's probably an alternating code - especially if the string looks particularly long
Type C, U and L codes - if you can't work it out by counting letters, it could be a mirror code (AZ, BY etc.)
Type B, D, E, H, Q, S - work directly from the answer sheet.
Type Z questions - circle question number and leave to the end (I also tell mine to turn down the corner of the page
so that they can find it again quickly)
Also regarding type Z - It's a good idea to establish what information is actually needed before starting on the diagram. There's no point itemizing all the specific things purchased in the restaurant ("e.g. Catherine ordered bacon and beans, William ordered a cheese roll, etc. etc." ) if the question only asks who bought the most items, for which a simple tally will do.
Type Z (again) - if it's the type that says "only one of the following statements is true", once you've identified that statement, there's no need to try the other possibilities (unless there's time and you want to double-check).
If you really can't answer a question, circle the number on the question sheet (and fold down the page corner so you can find it more quickly later), mark an answer anyway (see below) and move on.
Finally, if you absolutely have to guess any answers after working through all your strategies (eliminating wrong answers etc), then choose all of the same
answer rather than randomly selecting. This applies whether you answer a block of questions at the end or individual questions during the test. I have found that answer "D" (or the 4th answer) seems to work best for this, for some peculiar reason.
These are just a few off the top of my head, I'm sure you'll get more suggestions. Mods - would this be better under Verbal Reasoning?
Edited to add: agree with Chicko-Mum, some great tips in those threads (I think I picked up most of mine there!