When planning your trip choose a minimum stay of 2 or three days or spend a night from Saturday to Sunday at the destination. For some destinations minimum stay can be 5-7 days, so play a little bit with departure and arrival days to get the lowest price.
Maximum stay is essential too, if this is not a city break. Maximum validity of any airline ticket is one year, but the fares usually have less validity. For long haul flights this is usually 1 month. For short haul flights it differs a lot depending on routs and airlines.
Try to compose your itinerary so that all flights are operated by the same airline or choose its code share flights. If you still want to include other airlines in some complex itineraries, choose a pair of return flights operated by that airline instead of one-way flights.