I'm on OCR for all three sciences and have been doing the unit 3 ISAs over the past few weeks. Haven't received any feedback about chemistry but I know I've done well in the physics ones, so I'll try and give you some advice.
Make sure you can draw a graph - it's not just about plotting the points accurately. Remember that a line of best fit is one where 50% of the data points are on one side and 50% are on the other. Your axis must be labelled - if they gave you a table earlier in the paper, use those EXACT headings to label the graph. Do not forget your units. You can either use brackets () or a slash / to indicate units on your graph, e.g. "Length/m" or "Speed (ms^-1)".
The independant variable goes on the x-axis - this is the thing you are changing each time you take a reading in the experiment, such as the length of a rope or the weight hanging from a spring. The dependant variable is the measured result of the experiment - the potential difference reading your voltmeter gives you, or how far the ball rolled, or the time taken for something to happen.
Convert your units to SI before any calculations - if you are given measurements in cm or mm, convert to metres. Use decimal places to indicate the accuracy of your measurements - 20cm is 0.2m but if you measured it with a ruler that goes to mm you know it's more precise than just 0.2m, so it's better to put 0.200m instead.
Know your units, and know what they mean - resistivity is measure in Ohm metres, not Ohms per metre, for instance.
Remember things from GCSE maths - the cross-sectional area of a wire is a circle, so you can use the formula pi * r^2. Don't forget to convert to metres BEFORE you do that! Also don't forget to halve the diameter to get the radius first. Remember that the formula of a straight line on a graph is always y = mx + c where m is the gradient and c is the y-intercept. To find the gradient take two points on the line (x1,y1) (x2,y2) and do (y2-y1)/(x2-x1).
The number of significant figures/decimal places you give when you write down numerical answers should be roughly the same level of accuracy as the equipment you used in the experiment - if your ammeter gives readings to 3 significant figures and your voltmeter gives readings to 4, then 3 or 4 significant figures is acceptable in your answer - I would use 3 as generally you can go slighly less in your final answer.
Keep to the same number of decimal places throughout columns in a table - if your first reading was 0.32 and your second was 0.4 you MUST write it as 0.40 in order to remain consistent. Speaking of tables, if you have to draw your own, think very carefully about how many columns you need and how much space you will need to record your results. Use as much of the space provided as you can so it is clear - this goes for graphs as well. You can use pencil for graphs/tables for OCR as far as I know.
Show ALL the steps in your calculations. If you measured something three times and took an average, WRITE DOWN that you measured it several times, show that you calculated the average, show if you then rounded that average.
Even if an experiment doesn't require you to do repeats, it may be worth it to check for errors as you are going - unless it will waste lots of your time to take readings twice.
READ THROUGH THE WHOLE EXAM PAPER BEFORE YOU START THE EXPERIMENT. That way if the last page of the paper asks questions about the physics behind the experiment, you can think about this as you are carrying it out and it will be much easier to write a coherent answer when you get to the end. If the paper ends with a 4 mark question with half a page to fill, stop and think about exactly what points you want to get across to answer the question - don't waffle.
If something goes wrong, don't panic. You should have lab technicians and teachers who will help you clear up and get set up again (I knocked over a beaker of chemicals in a chemistry ISA, had to start all over again and waste twenty minutes because you had to leave a sample in a test tube for the teacher to mark. Our lab tech helped me clear my desk and I took a deep breath before starting again, and just managed to finish the whole thing in time).
If I think of anything else to add I may edit this. If you have any more questions go ahead and I'll see what I can answer (obviously without going into detail about ISAs we've completed, I have been very careful here not to give away any such information). Good luck!