Hola llias,

With time phrases like: "en los 'ultimos meses", "en los 'ultimos años", we see ourselves still inside of that time frame. If I say this now:

"He salido mucho en los últimos meses"

I am thinking of myself still being insde that, so,  "the last few months" is including the current month (march) and I am still inside it, so we tend to use the perfect tense in this case. 

If I had used the preterite instead (salí) it is likely that the listener is going to think I am talking about the last few months up to now and is not including the current month. In this case, we see ourselves outside the time frame and we are disconnected from it, triggering the preterite "Salí".

I hope this helps.

Saludos, 

Inma