Try using the same weight. That seems to help me. Doing more sets with a medium weight.
This is good advice, try 3 sets of 8-12 reps @ 70-80% of your 1RM. Keep the rest around 1 minute between sets.
IMPORTANTLY: If you're working out the same bodypart multiple times in a week, try a few times in the week to continue to lift the weight at the same max intensity as before.
Ex: If you're pyramiding up to 100lb dumbbells on db press for a set of 6, then starting now you can go ahead and get warmed up then knock out a set of 6 with the 100 lb dumbbells, and for the next 3 sets shoot for 8-12 reps with the 70-80 lb dumbbells.
If you're used to pyramiding up to the 100 lb dumbbells for sets of 6, then you switch to lower weight but higher rep routine, ultimately you will lose some strength. If this is the case then the next time you get back to a pyramid routine and try the 100s, you won't be able to get as many reps as before.
Remember that the most "unforgiving" element of training is your intensity. Go ahead and knock out the first set at your current training intensity, just to preserve your hard fought strength. Once that set is out of the way you can feel free to experiment however you wish: whether it is a reverse pyramid or same weight for multiple sets (aka static load).
Now, conversely, you may be in an overtraining rut. Consider taking off a week or two off if an honest self evaluation says you're overdoing it.