This is the layout i always followed. It's never failed me.
Firstly, ALWAYS WRITE IN THIRD PERSON, PAST TENSE. IE: "The test tube was then put in the beaker of boiling water, and heated to 80 degrees celcius."
NOT: "John put the test tube in the beaker of boiling water, and in the process, burnt his hand and killed 3 small house cats."
Aim - What you aim to get out of the experiment. IE: To find out the effects of putting 25000 volts through a science teacher', or 'To find the impact resistance of a footy jock's head to various hammers.'
Hypothesis: What you think the result will be: Ie: "Mr Teacher will not be able to handle 25000 volts, and this will be apparent when he is reduced to ash from the practical."
Materials: What the hell you use. Do in point form. ie: - Large fuckoff power supply.
Method: write in 3rd person, past tense. always. In complete sentences. ie: Firstly, the probes were hooked up to the teacher. Then, the power supply was turned on. All observations were recorded.
Results: List a table of results. Do not discuss them here, just list any results obtained.
Discussion: This is the marks winner. Make it the major written part of the report. Discuss why you got the results, and if any errors, why those errors could be. absolutely rape the ability to talk about inaccuracies in equipment to justify slightly wrong errors. IE: "Despite being unexpected, when 25000 volts were applied to the teacher, he managed to sing "God save the Queen" in latin. This can be put down to the fact that in a past life, he was once the Prince of Egypt"
Conclusion: Refer to your hyptohesis, stating whether it was proven or disproven. Also state whether the aim of the experiment was fulfilled or not. Should be no more than 3 or so lines.
I know it's a brief rundown, but this always won me my marks. If i can be bothered, i may or may not put up an example from one of my practicals that i did.
Good luck.
Edit: read T-I-M-M-Y's post. his method is pretty much the same as mine, and covers points i forgot.
Edit#2: rephrased aim to be statements, not questions, ala: helper's point