How do I patch a tube's ribbed area? Normally, when applying a patch to a tube, both the patch and the tube surfaces are smooth and it is easy to press the patch onto the tube.

However, some tubes have small ribbed areas where it is hard to get a patch to fully adhere.

What do you do to avoid small air leaks by a rib?

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Recently, I had another leak in the ribbed area so I took the advice and used a smaller patch, scratched things up better, waited 5-10 minutes, and pressed down with my full body weight. The patch lasted about a week before developing the following slow leak.
More careful buffing - the ribs stop the sandpaper/scratcher from getting into the butyl rubber and exposing raw rubber, so the vulcanising agent can't work so good.  

You don't need to buff the ribs off completely, but scratching up beside the ribs helps a lot. 

Also, over-size the area that you cover when you spread agent.  Those orange edges should be stuck down firmly.

Another help is to use a larger patch in these areas, but it means more buffing because more surface area.  You can also rotate your patch 90 degrees to cover the length of the ribs more.

Also make sure to wait 5-10 minutes after applying vulcanising agent, before pressing the patch on.  Then press patch firmly.  I use a roller the width of two coins, but a large round coin works fine too.