Welcome to The Shadow's primer on public pugilism.
If you're involved in Muay Thai and BJJ you're off to a good start. Both are excellent , mostly practical arts. Depending on your BJJ school and whether they focus on just the ground game or other aspects, you may wish to look into Judo or Japanese Jujutsu/Aikido/Chin Na/whatever for some throws and joint locks. Just keep in mind that Aikido and many Chinese MA schools go heavy on the woo-woo stuff.
Most of the time, you're better off trying to escape than to fight it out. If you must- use any advantage you can. Fair fights are for boxing rings.
Whether you're striking with fists or flat hands, keep your fingers together. A stray finger can be wrenched in a lot of painful directions. Do not under any circumstances do the MovieBob punch where you tuck your thumb inside your closed fingers. Unless you actually are MovieBob, in which case go ahead and break those thumbs.
If you get angry, your opponent has the upper hand. You want to stay as clear-headed as humanly possible through any fight. By the same token, if you can get your opponent angry they will be stupider by a good degree. Not that you necessarily should do this, but anger is the enemy of clear thought.
Contrary to what many BJJ promoters will tell you, it's not a great idea to take a fight to the ground in the wild, especially if you have multiple opponents. You go to the ground with one guy and his buddy starts kicking your teeth in, you're done. To say nothing of what the condition of the ground will be- pebbles, broken glass, all kinds of things can give you pain on the ground.
To amend the physical fitness thing- endurance is important in a fight. Running is a good way to get this up, and either working the bags at your gym or shadow boxing vigorously can help, too.
In hand to hand combat, a melee weapon massively changes the dynamic of the fight. Someone pulls a knife it immediately makes clinching and striking that much more dangerous for their opponent. I am not saying carry such a weapon, because that can be legally construed in some unfortunate ways even in clear cut cases of self defense and is often subject to different carry laws than firearms in different jurisdictions. If you do decide to carry one, do the legal research first. And a pocket knife without a locking or fixed blade is as much a danger to its wielder as to the opponent.
A group melee is significantly more dangerous. In a one on one fight it's best to never take your eyes off your opponent- with multiple attackers you've got that much more to worry about. But using your environment well can help you here in some cases. If there's anything you can do to make it more difficult for them to get to you- kick a trash can over, knock a table or chair into their path- you can at least try to manage your opponents better.
Also, I wouldn't be The Shadow if I didn't recommend cackling maniacally while taking your opponent down. Total power move.
