Most airlines have a loyalty program (a.k.a. "frequent-flyer program") where you earn points from flights you take and those points can eventually be used to book free bonus flights. Effectively you can end up with "every tenth flight is free" for some value of 10, though predicting exactly how many flights you need to take to qualify for one similar bonus trip can take a lot of prior research.

The downside (and the reason airlines do this) is that you need to make all your trips with the same airline (or at least the same alliance), so you will be less able to take advantage of better offers from other airlines, lest you end up with points in different loyalty programs that you can't combine into a flight.

Apart from this, there's not much you can do, other than book your trips well in advance before the cheap seats sell out.

It used to be that you could save significant money by buying a series of "back-to-back" Spain-Netherlands-Spain round trips with a stay of one month each, rather than a series of short Netherlands-Spain-Netherlands trips. However, pricing strategies that reward longer stays in this way have become less common. It is still worth looking into.

Also remember that flights with an connection can be significantly cheaper than direct flights, so don't limit your search to airlines that fly directly between your destinations -- unless, of course, the time saving of direct flight is important enough to you that you're prepared to pay a premium fo rit.

Most airlines have a loyalty program (a.k.a. "frequent-flyer program") where you earn points from flights you take and those points can eventually be used to book free bonus flights. Effectively you can end up with "every tenth flight is free" for some value of 10, though predicting exactly how many flights you need to take to qualify for one similar bonus trip can take a lot of prior research.

The downside (and the reason airlines do this) is that you need to make all your trips with the same airline (or at least the same alliance), so you will be less able to take advantage of better offers from other airlines, lest you end up with points in different loyalty programs that you can't combine into a flight.

Apart from this, there's not much you can do, other than book your trips well in advance before the cheap seats sell out.

It used to be that you could save significant money by buying a series of "back-to-back" Spain-Netherlands-Spain round trips with a stay of one month each, rather than a series of short Netherlands-Spain-Netherlands trips. However, pricing strategies that reward longer stays in this way have become less common. It is still worth looking into.

Also remember that flights with a connection can be significantly cheaper than direct flights, so don't limit your search to airlines that fly directly between your destinations -- unless, of course, the time saving of direct flight is important enough to you that you're prepared to pay a premium for it.