Having just moved to a city that where I don't know any other rider, riding by myself is a common occurrence. I definitely change my riding style and try not to take unnecessary risks. I tend to leave the doubles these days, and shoulder the bike over sections with shear drops off to one side etc. I don't shy away from riding in remote places and riding technical trails but I do make sure that I don't go any further than about 10kms from the road end (aka a two hour hike out).
But as others say, be prepared. Take food, water, a first aid kit and compression bandage (thinking about it, wouldn't a spare tube could also make a good compression bandage should you get a snake bite? keen to hear what peps think about that)... Basically I make sure have enough with me that should I have a bad off or a serious mechanical and are forced to /can walk out I don't get heat exhaustion or dehydration.
And as for contact, I tend to ride areas that have decent phone coverage if possible. And I do take with me a charged smart phone, so I have the ability to tell emergency services my GPS coordinates. I don't personally have a tracker app on my phone but I am thinking of getting one just in case, so my wife can pass on these details to SES should I not return from a ride.
But at the end of the day, riding by yourself has increased risks. For instance my uncle was working by himself in the bush, slipped, knocked himself unconscious and fell face down in a puddle. He drowned in 2" of water. Terrible luck I know but if someone else had been there he'd be alive today. Food for thought.