Time > 1 Year-
Most important thing is to read all the standard books which are prescribed, go thoroughly. Have some patience ,there are some topics deserve much more time(6-7 times reading) so in that case patience is the key.
Next thing is to improve your problem solving ability , Irodov is a wonderful book which will help you to do so in every branch of physics ( it'll also help you to brush up your basic concepts).
Last solve as many test-series as you can, preferebly 5-6 test series and around 100 test papers in total in last 4-5 months ,that playes the most important role in acheiving double digit rank. Many of them are provided freely here too.
Time = 1 Year- Stratergy is nearly same as above, just do thing fastly but don't compromise with quality. Narrow down your study material.
Time ≈ 6 months-
Solve/Analyse all previous year questions, but more importantly analyse what number of questions asked from a particular branch and prepare accordingly.
Next thing is to Make short notes by yourself, write everything in it which you think is important. Revise them daily, twice on Sundays.
Don't touch whole new topics in last 1-2 months , solve as many tests as you can in last 3-4 months. After solving tests analyse your mistakes and your weak topics and work on them.
Time < 3-4 months- It's really important that you find scoring topics and go after them. Pick up any coaching material and solve those jam related problems. Don't try to cover all syllabus, make 5 subjects strong and solve their Unit Tests also.
I don't think coaching is really necessary for iit jam, even AIR under-10 is surely possible without any coaching. See examples. (click here)
It doesn't matter how you much marks you got till now. If you decide today, decide now, that I'll change and give it my all. No one can stop you to from conquoring this challenge called JAM.