How do you prevent yourself from yawning and falling asleep while you meditate?

Just try to have a straight back during your practice. It's ok to not be fully comfortable in order to be in the correct state, which is different from sleeping 😛
Have a nice YOU time!

In addition to sitting up straight, I sometimes keep my eyes open and gaze gently at a blank wall. I also moved the meditation habit earlier in the nighttime ritual so I wouldn't be quite so sleepy at the start of it!

By focusing on the feeling of my body and my breath, I stay focussed and alert. It's also important to follow a regular sleep routine.

I sometimes do and I find it good that I reached a state of relaxation that made me able to go to sleep with no much effort. I sleep for seconds though and wake up immediately once the tone of voice change and then realize I am relaxed and in a good mood

I used to fall asleep at the beginning of my meditation journey…it takes time, patience, and practice. Like any new skill, it takes practice to master/grasp it. Be gentle and kind with yourself. Keep at it, and you will eventually find your center, and be able to meditate without falling asleep.💚

Ik zorg dat ik in een comfortabele houding zit. Ontspannen, maar niet relaxed liggend. Rechtop, maar zonder mezelf in een rechte houding te forceren. Op deze manier ben ik relaxed, maar niet zo ontspannen dat ik slaperig word.
Daarnaast doe ik een guided meditatie voor focus en hulp en focus ik op mijn ademhaling.

I find myself thinking that I've never felt sleepy when I meditate. This is because I always try to meditate with the knowledge that I am well rested enough. I think if you feel tired as you are meditating, then maybe you should give yourself some time to rest a bit more. This is not a criticism on your methodology or approach to meditation, it rather sounds to me like your body is trying to tell you something. Maybe you just need a bit more z's and then after that you can meditate to bring your mind at ease.