How to do Two Sided Printing on a Single Sided Printer

How

This saves paper and halves the bulk of printed documents to carry around. It is fairly easy:

If the printer naturally outputs pages in the correct order (typical for a laser printer which gives out its sheets in a face-down pile).
1. Print only the even numbered pages, printing in reverse order.
2. Put the stack of printed paper back in the fresh paper tray.
3. Print only the odd numbered pages, printing in normal order.
If the printer naturally outputs pages in reverse order (typical for an ink jet printer which gives out its sheets in a face-up pile).
1. Print only the even numbered pages, printing in normal order.
2. Put the stack of printed paper back in the fresh paper tray.
3. Print only the odd numbered pages, printing in reverse order.

The only tricky bit is getting the paper in the right way round when putting it back in. Getting it the wrong side up will cause overprinting and getting it the wrong end up will cause half the pages to be upside down. The correct way up will depend on the particular printer.

Most small laser printers these days take paper in from a fresh paper tray at the bottom, fed in short edge first, print on it starting with the page top, flip it over top to bottom and spew it out above the fresh paper tray. For these, put the even-side printed paper back in the fresh paper tray face down with the top of the page pointing towards the printer.
Most cheap inkjet printers these days take paper in from a fresh paper tray at the top, fed in short edge first, print on it starting with the page top, and spew it out below infront of the printer. For these, also put the even-side printed paper back in the fresh paper tray face down with the top of the page pointing towards the printer.

Anyway, it only needs testing once for a printer, there are only 4 possibilities to test and the sheets of paper consumed in testing will immediately be compensated for by the first pages of two-sided printing.

Of course, if your document is formatted with any blank sides (typically blank even pages to ensure sections start on facing pages), ensure these are printed (or manually insert blank sheets) so that the second printing has something to print on.

Problems

There might be some risk of jamming (but this is only from not neatly restacking the paper before putting it back in), sticking (let laser printer ink cool before feeding the paper back in) or of water soluble ink jet inks soaking through the paper and dissolving the ink on the other side (but if your printer is doing that then there is a problem to be cured anyway before you waste too much expensive ink!).

It is hassle to keep putting paper back in for short documents (especially because it gets confusing if you are queuing lots of documents in the print queue or what to quickly close or edit a document after sending to the printer) but for big documents, I find it more hassle to have to carry around (or find filing space for) twice as much paper as necessary afterwards.