I used to ride a Sportster, and I'll tell you what worked for me: (1) Get a padded "overlay" grip for tennis rackets. It's meant to go over a regular leather tennis handle grip, it's self-sticking & there are usually two in a package. Put it right over whatever base grip you have on there now. (2) Make sure the overall circumference is not too small for the size of your hand. The smaller it is, the tighter you squeeze & the more you cut off the capillaries trying to return the blood out of your fingers which causes most of the tingly numb feeling. (3) If all else fails or if you're riding beaucoup miles over several days, buy fingerless gloves with padded palms from a bicycle shop. They come in all colors & you can even get "bike" leather on the top side if you shop around. (4) Make sure your gloves are not too tight, especially around the knuckles.
Sportsters are very fun rides, so keep experimenting until you find a solution. And when your butt starts going numb, take another hint from those crazy Tour de France bicyclists and invest in a high-end gel seat!