The key to keeping a travel journal is to make sure you never feel like writing in it is like homework. Use whatever technique works best for you. I’ve used various techniques from keeping very detailed travel journals for longer trips to keeping a simple log of events for a shorter trip. One journal of mine is in the form of a calendar; I returned home scribbled down what I did on each day I was gone. It was not my best travel writing, but the events immediately following the trip were much fresher in my mind at that point than they would be if I now tried to recall details after years have passed.
Of course my favorite journals to look back at are the ones where I included lots of details. In these, I jotted down descriptions of some photos I took (words to go along with the pictures), included inserts such as tickets from miscellaneous boat rides, local maps and brochures, and included the odd menu from my favorite local eatery. I’m not much of a sketch artist, but I’d recommend drawing a sketch or two if you have the knack for such things. I’ve collected recipes from abroad to slip in my journal to discover upon my return. I’ve written the names of people I’ve met, words I’ve learned or phrases one might not come across in a phrasebook. I do always try to include an actual log of where I was on what day and where I stayed along with my first and lasting impressions. Since I do not often travel in a very planned or organized way, my travel journals tend to take on the same pattern.
Try to write frequently enough to create a memory, but not too frequently where you feel like keeping the journal updated feels like work. The great part of keeping a journal is that you can keep it for just yourself or you can choose to share it and perhaps even save it for your children or grandchildren to read. I just came across a journal my grandmother kept on her overland journey from Turkey to Egypt in 1980 at the age of 60. It was fun to compare her experiences with my own experiences in the same area almost two decades later.