IMO, panning is one thing, it' great for helping certain elements to sit more in the mix or to create some interest like with hats & such elements i.e. Now depending on your Daw and your source material & channel, panning may just result in moving elements to the L or R channels, it's not affecting the stereo spread of those sounds.
Tools like the free iZotope Ozone Imager of Flux Stereo Tool can help to expand the stereo field. Using them per channels is an option, or you could use send channels. Other Fx that can widen the sound are delays and reverb.
To check your stereo image, Flux Stereo Tool can help too and HoRNet Plugins has also got a free StereoView plugin ( I think you just need to give an email address to get it ).
Now another thing than can help, either per channel or on the mix bus is the use of Mid/Sides EQ, basically you can set a low frequency cut point that you want to keep mono on the Mid EQ curve and on the Side one you can boost higher frequencies slightly, this can also help to widen the mix with retaining a certain amount of mono compatibility.
M/S Equing is quite tricky, you have to find the right balance between Mid & Sides and it can help to use a glue compressor after it too, but yea, things to play around with I think.
Here are the links to those free plugins and BTW if you haven't checked it yet, latest Protocuture tutorial on EQ Vol.2 is a great learning resource, one to watch for sure.
EDIT : Forgot to mention another plugin that might give some "instant" result, this one is a payed one, but very affordable, it's GROW by Mastering The Mix. They do great stuff too BTW. This one could help per track or on the overall mix bus also, you can demo it as well.
EQ Tutorial : https://www.sonicacademy.com/courses/eq-level-2-with-protoculture