1) The allies considered D-Day in 1943, and a 1943 operation could have easily failed.
2) If you just want a few month delay up to D-Day, have the USA delay torch. Not all Generals supported the operation. So if Torch is launched 6 months late, the USA is behind on the time table in the south, but D-Day goes on schedule.
3) Increase U-boat success. Less supplies = slower operations.
4) Kill the P-51 development program, so the Luftwaffe is beaten a bit slower.
5) Germany build jet fighters sooner, take longer to lose control of airspace.
6) If just want to slow down Sicily, have Patton get fired sooner. Mark Clark tended to be a more cautious commander, so you might slow things down by a few weeks. Really, any major shake up of command staff right before a battle could slow things down, so pick a reason to fire/kill a bunch of Generals. For example, in North Africa in a meeting between Patton and his staff with Monty and his staff, an air raid kills most of them.
Is this for a TL? Did you have something particular in mind?