This is pretty last minute, but I just discovered this yesterday and thought it would be a good thing to share in case anyone else wasn't aware.
Until tax day (April 15th), you can still contribute to your 2018 HSA (up to your max), and get a tax refund from it. You can even do this if you've already filed your 2018 taxes! Of course, in that case you will need to file an amendment, but the savings are absolutely worth it.
I started learning about all this stuff just recently, and had only contributed ~$50/mo last year, so I just made a $1,500 contribution, and I'll be getting over $400 back in free money!
Now, moving forward, you should obviously max out the HSA through payroll if you can, since you also save on FICA that way, but this is an awesome opportunity to capture some extra tax benefits before it's too late.