1) write about what you find important/interesting
2) write/observe accurately
3) don't bother about making things readable/attractive for everyone; lab journal writing serves to give you alternative ideas about a problem, strengthen your memory and free space from your working memory.
4) feel free to write about your feelings, your dreams, even little poems if you feel like it; if something is in your mind, write it down so you can get on with other business without it needlessly cluttering your brain.
5) when you feel bored, or for whatever not able to concentrate on the writing, stop and go do something else for a while.