Why do I keep getting flats? I only bike for pleasure and sometimes to get around.  I know very little about bikes.

A few months ago I got a B'Twin mountain bike.  Where I live now I do not feel safe biking in the city so I only use it on the weekends when the weather is good to get out to nature (on some otherwise clean looking dirt roads where many people bike).

Despite the light usage I already got a flat three times.  In the past I used to bike to work every day (with a similar but simpler bike) and I have never gotten a flat in several years.

What could be the reason for all the flats and what I can do to prevent them?  The first time I got a puncture I took the bike to the shop to get it fixed, so I don't know what caused it.  The second time I thought it's time I learnt how to do this myself, so I changed the inner tube alone.  I found a thorn.  The third time I found a small piece of metal wire stuck in the tyre, which may or may not have been the cause of the tiny hole in the inner tube.

I don't have great job stability so I expect to have to move often (1-2 years) and unfortunately I cannot keep a bike for a long time.  Thus I am looking for a solution which is not too expensive.  While many people will consider this bicycle quite cheap, for me it was the most expensive bicycle purchase so far ... as I said unfortunately I can't make a long-term investment.

Am I getting flats because I go on dirt roads too often?  

Or is it because of the quality of the tyre?  Should I change that?  I can find different compatible tyres between 9 EUR – 40 EUR and practically all are advertised as puncture resistant, so it's not clear to me if there is a significant difference between them.  What I care about mostly is avoiding flats, not high performance (low weight, etc.)

Should I consider a "puncture proof" inner tube instead, with the slime inside?  I read mixed opinions about those and it seems like it's not the best choice for my situation.

Should I consider a tyre liner instead?

To sum up, I am looking for an affordable and simple way to reduce the frequency of punctures.  I need an affordable way because unfortunately I may have to sell the bicycle in less than a year.



Notes: I keep the pressure around 50 psi; the tyre has 35-65 written on it.  I read this post where the ultimate solution was to change the tyre, but it's not clear if it's also the right solution for me, and it's not clear if I really have to go for an expensive tyre go get useful puncture resistance, or if the standard B'Twin brand will do just a well (even if it otherwise offers lower performance).
I own a bike shop and one of the reasons I noticed when a tube is punctured is that the rim tape/strip is not in place properly, like it has folded over itself.  If you have ridden your bike for a while, the rivets on the rim may have rusted and they create sharp edges that can puncture your tyre.  If this is the case, straighten the rim tape/strip to cover the rivets completely.  You will have to change the rim tape/strip if that is no longer feasible.  If you would like to save on paying for it, ask the bike shop to give you a punctured tube of size smaller than your wheel size.  You can cut a strip off the punctured tube and place it over the rim.  Make sure that you have cut a hole big enough to push through the tube valve.