Keep your tyres pumped up. A stand pump with a pressure gauge is best for this. The required pressure will be embossed on the wall of your tyre but if you can deform the shape by pressing it with your thumb it’s too soft. I prefer to work in PSI (Pounds per Square Inch) than Bar as there are more of them so it’s easier to be exact but, considering that the “ideal pressure” is a range, exact isn’t something which really applies here anyway, at least not at my level.
The softer your tyre, the more comfortable the ride will be, but it will be harder work getting there as you will have more rolling resistance from the road to overcome, plus you will be constantly putting energy into deforming the shape of the tyre as it rolls along the road. The harder your tyre, the bumpier the ride will be but it also will be both easier and faster.
I need to pump my tyres more often, I need the speed at the end of the day and I have padded shorts to cope with the bumps.
Check your tyres for bits. Little stones, pieces of glass, thorns, things like that will embed themselves in the rubber of the tyres. A colleague of mine checked one of my tyres a couple of months ago, showing me what to look for, and I’ve done it every so often since then. Yesterday, a different colleague came across me as I was checking my tyre and told me a much easier way of doing it.
Let most of the air out of your tyres. Pick a start point (the valve or reflector or another distinctive marker) and work your way around the tyre. Pinch the tyre between your fingers, rolling it a little if necessary, to open up any cuts, and use a fingernail to pry out any debris embedded in the rubber.
Some riders I know do this before or after each ride (probably not both) but I think once a week is probably sufficient for me (it will depend on what kind of mileage you do). In reality I do it around once a month but yesterday I was amazed I hadn’t had a flat considering the amount of rubbish in my tyres!