It's definitely easier with a 3rd party, but here's what we did. Note that before this experiment I hadn't had any caffeine at all for several years because I was pretty sure it affected me badly.

(1) Experimenter dissolved measured weight caffeine powder in water, and divided it into various measured portions, and labeled them with unique numbers. Then made identical measured portions of pure water, also labeled. She sealed the number assignments in an envelope.

(2) On experiment days, I chose a water portion, recording the day and number in secret - she could not see which portion I chose or what number it was. I then poured the water into whatever (zero caffeine!) drink (or whatever else I could mix it in with) I was having. I used drinks and food with strong enough flavors that I definitely could not taste the possible caffeine addition.

So now neither of us knows whether I took one of the caffeine ones.

(3) I recorded the day's results.