Normally when I use my headphones [Monoprice DJ8323] I just place them on my head in a way that feels comfortable. Just a short while ago I started playing with them while listening to a Nektar album and discovered that by turning the cups so that there is more pressure against my head towards the back of the cup and less pressure towards the front the SQ was significantly improved. More clarity and detail. I'm rather surprised that this minor adjustment makes such a big difference! A timely discovery too as I'm going to demo some better headphones later in the week.
I understand there's a new audiophile accessory to surgically shave down the sides of your head so that headphones will fit better. They add a green ink ring during the process, so everything sounds more detailed and precise, too. $99.95 at Earl Scheib.
Pressing on the back of the cup may have created a better seal. Getting a good seal with the ear pads does affect the overall sound. Getting a good seal around the bottom of the ear near the jaw can be a problem for some headphones depending on your jaw line and head shape. Pressing on the back of the cup may have sealed that area for you. More headphones now are using a wedge shaped ear pad. Where the pad is thicker behind the ear and thinner in front of the ear. Look at the Audeze headphones as an example. That gets the driver to be more in line with your outer ear. Seems to help get the sound to seem it is slightly more in front of you. Position of the driver in relation to the ear also affects the sound. Try moving the cups forward so the back of your ear is touching or getting crowded by the back of the ear pad. Also try moving the ear cups down so the top of the ear pad touches the top of your ear. Experiment. Positioning the ear cups down and forward like that can give you a better sound stage that is more in front and less of a blob between the ears. Helps to have large full size ear cups to do this. But even with smaller ear cups and on-ears you can experiment and get some benefit.
The Headphone Tips and Tricks section on the Meier-Audio web site has a picture and explanation of what I was describing about forward and down headphone position.
You could also jam some Mpingo Discs between the cups and your ears, even glue some on Shakti Stones.
That only works after you've left the headphones on your ears for the mandatory 1000-hour burn-in period.
Some headphone manufacturers offer a burn-in service for an additional fee. Well worth the money I'm told