Pancake holster, larger, slightly baggy or extra long polo shirt (the kind that are designed to NOT be tucked in, usually have a little v shaped cut along the hemline on left and right) DARK IN COLOR (white and light colors are going to show a silhouette of a gun much easier.
BIGGEST thing is mindset. If you walk around like carrying a gun is the most normal thing in the world, most people won't suspect the bulge is a gun (especially in this day and age where middle class white guys walk around with cell phones and other crap on their belts like they're Batman).
Standing Wolf (who posts here a lot) often open carries a .44mag revolver and has commented several times about going about his day and nobody notices the giant shiny silver pistol with light colored Nill grips on it.
DON'T TOUCH IT. Constantly patting the gun, checking your shirt, etc is just going to draw attention to it. Again, most people won't notice.
Also if you've got a gut and love handles, OWB is just not going to conceal that well.
Overall, a good IWB is still a better choice for concealing a handgun. If you get pants a size larger and have a good sturdy belt and wear a t-shirt under the polo (so you can tuck it behind the holster) you're going to find a good IWB holster is every bit as comfortable as an OWB ... maybe moreso because it will clearly conceal better.
Oh ...an after thought. Hawaiian type shirts are generally considered on the same level of casual vs formal as a polo shirt, yet they conceal better and are more comfortable in the heat ... so consider them. You can get Hawaiian shirts without garish prints on them ... but the garish prints do help conceal the outline of the gun (if my wife didn't outright forbid it, I'd wear nothing but Hawaiian shirts whenever the temperature is above 30)