For about ten minutes a day or longer find a small spot to focus on like the tip of a pencil or a small dot and keep your focus for as long as you can without being distracted and without looking away. This has significantly improve my mental sharpness, when I am talking to a girl I am totally present and all of my focus is on the situation and what is happening rather than having wandering or nervous thoughts. I remember the techniques better and I seem to know the right thing to say rather than thinking "oh shit what do I say now" and I avoid classic mistakes such as leaning in and focusing the conversation on myself which is probably the thing I am most guilty of.
The second tip for mental shaprness is very obvious...Get enough sleep! I cannot stress how important this is. When you are fully refreshed you are a lot more conscious of what is happening around you and you have a better natural energy. So if focus is your problem give these two things a go and I am sure you will see big changes just as I have.