Good start on being cautious with your addendum!
One thing I've learnt from overhearing conversations on the matter, is you should try to imagine how would you attempt to overpower yourself. What would be the most likely attack that someone would use against you, that would work well on you? Then you think of counter measures for it - things like always keeping one hand in your pocket, clutching your keys as a makeshift knuckleduster. Most of the time, you only need to graze someone before they flee and you're within your right to defend yourself so long as you don't use excessive force.
But keeping an eye on your drink and making sure that your meal(s) are never tampered with are good places to start.