I've been working in sketching and wireframing land for a while now, and I'm seeing more and more need to start growing my visual design expertise. Do you have any ideas on how I can go about doing this? Any good resources to use?
If visual design isn't your focus, or you're inexperienced, I'd suggest finding one or two design styles (or designers) you love and try imitating them. Copying others is a means to an end not something you want to do in your portfolio. It helps build your visual design "muscles". You'll reach a point where you'll stop copying those you admire and you'll start to develop a design 'voice' of your own.
Also get feedback from designers on things you're working on. Ask them what they think is working in your designs and more importantly, ask them what isn't working and can be improved (I do this every day). Seek criticism.
It always helps to have a problem to solve with something like this. Make up a client and project you can use your skills on.
I keep meaning to write a piece on this topic because I've been in the position you are and am now able to do make some production quality visuals - not a lot, but some.
I guess the essence of it is to spend lots of time among the visual designers you work with and understand why they make choose certain solutions and figure out how they did it. Then mimic in your free time. I would also recommend using Sketch to save you all the headaches you would get if you used Photoshop - or even better, use html and css if you are doing web work.