Assuming that yours are like mine, you should see a small clear plastic spool or bobbin at the lower corner of each shade. These can be used to adjust the tension. The way I do it is to remove the bobbin, (the shade will then fall all the way down). You should find a little hole in the side of the bobbin, thread the cords through that hole and refasten it too the wall, but not tightly. Take up most of the slack, then turn the bobbin until the cord become tight. With trial and error, you should be able to fine tune the tension, so that the blinds just stay up without being too tight to move easily.
Another common problem with these blinds is that the cords become frayed and have to be replaced. Here is a link to some photos I took of the cord replacement process.
Day/night shades
On edit, I see Orv gave you much the same answer before I got mine posted.