Hey there. I'm an American historian by training and a museum professional by trade. As such, I have worked a lot with current researching historians and their sources.

Some of the things they look for are:

1. Relevance - historians research specific subjects. If, in the future, a given historian wants to research the social environment surrounding Brexit, they probably won't look at your diary unless you wrote relatively extensively about it. Don't worry, though; write about your life and the things important to you. Someday down the line it will probably be researched.

2. Context - research means very little without proper context. If, in the future, a historian wants to research American views about gender and sexuality during the early 21st century, their findings will be vastly improved given the context of your own gender and sexuality (and whether or not you're American). On a more macro scale, if a researcher wants to learn about how American citizens viewed the candidacy of Donald Trump, they would probably want the context of your own political views, nationality, gender, class, etc.

3. Readability - they have to be able to read your diary. The absolute biggest thing you can do to ensure your diary's place as a future primary source is ensure it sticks around. As a museum professional, this is perhaps the area I'm most qualified to talk about. That means:

a. Write it on acid-free paper with acid-free ink

b. Keep it dry

c. Store it someplace at 60 degrees Fahrenheit with 50% humidity

d. Wash and thoroughly dry your hands before you handle it

e. Leave a note with the diary and/or in your will instructing your heirs to hold onto it until they can find a museum or historical collection that wants to take it

As an aside, I'll add that a lot of people write their journal/diary digitally now. Museums and archives aren't really sure yet what the best way to preserve digital files is, so if you choose to keep a journal digitally, I'd keep four copies. One on a hard drive in a safety deposit box, one on a thumb drive in the safety deposit box, one on a CD/DVD in a different location, and one on your desktop. That's usually pretty similar to how museums store digital artifacts.

I hope that's helpful! Also, I hope this answer is thorough enough to meet /r/AskHistorians requirements. If it isn't, mods please PM me and I'll edit until it's good.

I can't speak for the physical side of things. But I can tell you as a historian who primarily uses primary sources for research it's rather difficult to ensure that your diary will be a valuable resource for future historians. Although I admit I find it funny as I often have similar thoughts.

Generally speaking, much of what is used as primary sources today is, to put it simply, junk or trash. For example, there is a very talented historian at Rice University named Tani Barlow that does a lot of research exploring the depiction of women in Chinese ads in the 20s. It's fascinating work but the majority of her work uses sources that most people at the time would have considered completely irrelevant or uninteresting. However, they are valuable windows into the cultural space of the time period she studies. So even if your current writing may seem useless or irrelevant to future historians it may in fact be useful. At the very least, preserving such things will certainly be interesting to your descendants.