How can I inform customers about increasing sales prices?

Carefully ;)

Most places I shop do not cost price increases, you just see it on the item when you go to purchase it. Now if you have a regular clientele, and you send out flyers with merchandise, you can certainly put something there about a price increase. Most consumers understand the prices go up - we expect it actually, do not like it, but understand that is the way of the market. Most excuses for price increases are tied to the higher fuel costs - shipping is costlier and the cost has to be passed on to someone. The store owner cannot absorb all that. Consumers understand that.

Would not expect a sign on your window about prices increasing. Lol.

If the price increases are to take effect soon but not immediately then you could advertise or tell your clients/customers of the price increase and suggest that they buy now prior to this increase. Thus increasing your sales now which could help offset a possible drop in sales when the increase hits.