Do not use sedatives (downers) in combination with other sedatives (alcohol, GHB, sleeping pills, opiates) or ketamine. This is dangerous because you can fall unconscious and suffocate on your vomit.
Write down the time of intake and your dose, text the dose and time to each other or put it in your phone to keep track of it yourself.
If you feel that you are getting falling asleep, you can try to keep moving to avoid falling asleep. But beware; if moving takes a lot of effort, there is a chance that you may fall or trip, so be careful.
An overdose is more likely on an empty stomach, a full stomach increases the chance of vomiting.
Less is more; taking a small, extra dose later because you did not feel anything after your first dose is better than taking too much and having an overdose.
Ensure that the people around you are aware of what you are going to use and have been using.