Staying clean is important for the health and also for social interactions. It doesn't mean you have to take a shower every day, but you know, staying fresh enough. Luckily developed cities provides us with some options in regards of hygiene.
First, the public or free toilets. This one is the most common and you can find it easily.
You can also find some free showers especially if your town is close to the sea. Otherwise you will be able to find them into some petrol stations. There is also the possibility to ask at swimming pool if you could use their shower quickly. If the receptionist is friendly, then you're free to go and enjoy your shower!
Otherwise you still have the possibility to jump into any lake, rivers, dams, oceans.. (but just make sure to don't use any product that could arm the environment!)