Remember that the purpose of your logbook is for it to be an irrefutable diary of your progress, so that if it comes to a legal conflict, you can produce your logbook and supporting evidence to substantiate your claim that you were the first to start working on your invention. So all the tips that I describe below are meant to show that your log entries are authentic and have not been modified after the fact.

If you are using a paper-based logbook, make sure it is one that has pages that cannot be added or removed easily. In other words, the binding should be very secure. Also make sure that each page has a serial number and you should fill up each page chronologically. This is to remove any doubt that someone could have tampered with the pages in your logbook.

When you make entries, don't leave spaces between entries and don't leave blank pages between entries. This makes it clear that you had no intention of going back to fill in additional information later on. Also, never amend an entry or add footnotes to an existing entry; it may look like you are falsifying the entry! If you want to note an amendment, make a new entry in your logbook saying what it is that you want to amend, then explain which entry it is you are referring to. Only then should you go back to an earlier entry and add a signed and dated footnote saying to refer to your new logbook entry that explains your amendment.

Always record the names, telephone numbers and addresses of people you have contacted in the process of working on your invention. This is useful if you need them to corroborate your story at a later time. This doesn't mean that you should tell them about your idea! What I mean is that they can be called as witnesses who could testify something like "Yes, he bought 20 sheets of aluminum from me on the 20th of January", which could support a claim that your were building your first prototype in January.

You should also include all drawings that you have made of your invention. Photos, which are dated, may also be helpful. Once your invention has become a workable prototype, you should consider explaining it to a trusted person, and having the person sign in your logbook saying something to the effect that "he has received an explanation, on this date, of the information in the logbook". This trusted person can then be called on as a witness if necessary. Naturally, for this person to be a credible witness, it should be someone of good characters, has no run-in with the law, and does not stand to benefit financially from your invention. (i.e. a co-inventor wouldn't count, because he or she could be construed as having some financial stake in the invention, and might be tempted to give unreliable witness testimony).