What are the most common places for such a leak? The Valve Cover Gasket seems good? I seem to recall that there is a seal somewhere on the back of the head.

There is a half moon shaped rubber gasket (plug) under the valve cover gasket at the back of the engine. It can dry out over time and leak oil. It's used on the B23F eingine from 1983 and probably later engines too. jp

That small half-moon rubber plug was only used on B21/23 engines, but the B230 series (1985 on) had the blanking plug shown below in Lucid's photo. The B230 as used in the 700/900 cars had the distributor mounted at the rear of the head, driven by the camshaft. In the 200 cars, the distributor remained on the left side of the block and the opening at the rear of the head was sealed as shown. It's a known leak point, and the retaining plate is a good thing, whether IPD's or DIY.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

If it's that round seal/plug at the back of the head, be careful. It can come completely out and cause big-time oil loss really fast. There isn't much room, but you can feel back there with your fingers to learn what's going on. Easier when the engine is not hot.

If you need to replace it, consider pulling the valve cover and the rear cam cap to access it. It's easy to replace. That retainer the other guys mentioned is a great idea. One our 86, the head was already drilled and tapped for it.
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Thanks everyone for all the information and advice, Doug C. 81 242 Brick Off Blocks, stock, B21F (non-Turbo), M46; 86 244, B230, 140k , auto.

Here's a picture of Don Foster's version of the IPD retainer that Jim mentioned below.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

Actually it's a large plug; ipd sells a retainer to keep it i place.
http://www.ipdusa.com/ProductsCat.aspx?CategoryID=95&NodeID=4991&RootID=629