Gather the necessary information on the route and adapt the timing to each one's personal physical condition.
It is highly advisable to know the weather forecast, because summer storms in the afternoon are quite frequent. If this is the case, and if there are lightnings, do not go under trees nor walk up the hills.
Tell somebody the route you are planning to take, specially if you go on your own.
Wear boots that fit well and are appropiate for the kind of walk.
Carry just the basic stuff in your rucksack: bag for the litter, water, waterproofs, sun cream, hat, food, map, guide, personal documentation, emergency kit, toilet paper and mobile phone (although some areas still have no net coverage).
In summer it is a good idea starting early in the morning to avoid the hottest hours of the day or, if it is a shorter walk, try going out in the afternoon.
Respect the crops, fields, and herds; if you happen to open a gate or wire for animals, make sure it is closed again before continuing.
Do not throw any kind of litter, even organic, specially cigarette ends or toilet paper. Put it all in your litter bag and soon you will find a rubbish container to get rid of it.
Emergency number (accidents, fires, robberies...): 112