It is no doubt that you need a lot of time to write a good lab report. First of all, you have to be very alert on what had happened during your experiment and take down any tiny observations that may be needed in your lab report even the abnormal or wrong things. Correct data must also be taken. Extra care given during the experiment time will directly help your report in all ways.
The most important thing is that you can accomplish the basic requirements of a report. The followings are the essential elements in a standard and simple lab report that is enough for you to win a good image from your teacher.
Experiment Ref. Number
Date
Title of your Report
Objective of the experiment
Theory involved, equations
Chemicals and Apparatus used
Procedure of experiment (in past passive voice)
Diagram of Apparatus / Set-up
Data taken during experiment (in table form)
Observations
Calculations involved
Experimental Results
Discussions (most important!)
Reference / Appendix
It is interesting to note that in writing a report, past tense is recommended as you are asked to report what you have done. Also, in describing the procedure, remember to use passive voice. For the observations, it must be clear and concise (e.g. the solids are completely dissolved in 5 seconds). Special attention should also be given to the significant figures in writing the data.
For the discussion part, it should be what teachers are specially looking for. You should discuss everything that may or may not be related to the experiment (such as aim, method, modification/improvement, procedures, or even what you have done wrong!). A handful of descriptions on the theory involved MUST be included. It is the best that you can add diagrams and reorganise the information you got from your reference book rather than just copying them out. The worst thing to do is copying your classmate's report which will be easily spotted by teacher.
Enjoy doing experiment! It's the best way to learn!