Do a google search on vocabulary memorization techniques, especially for languages without Chinese characters.  It's a common problem with plenty of solutions out there.  More exposure is always a good thing, but not the ideal solution; that's just brute forcing it.

Nuriko's idea is one of the better methods out there.  Being creative with sound association is another option.  Whatever you do, try to get your imaginative memory involved.

If you're having trouble with both of them you might want to try "dropping" one and not worrying about it until the other one's firmly in your memory. Then you can bring back the other later. Sometimes I find that trying to remember two similar words at the same time means I just remember them as a pair which I then can't disentangle.

(しっかり is easy for me because of the scene from Azumanga Daioh where Osaka repeats it over and over; すっかり I didn't even know before this thread; my impression is that すっきり is more common but perhaps that's just because I happen to know the word.)