The best stuff to use on them is Dubbin. It soaks into it, waterproofs them and turns them into that nice dark shade.
When I first got mine I spent about a week or so coating them in dubbin, lettignt hem dry, then coating them in dubbin again.
However, not all boots were Dubbin-ed. Some troops used Polish, some a mix, some even nothing.
Not polish, no. You will need to waterproof them. One way is to use a boot sealer spray of the sort you get in camping shops (such as Nikwax) which will retain much of the 'newness' of the roughout leather. But I'm an advocate of dubbin (which is fat/grease/wax) which is efficacious, cheap and easy. It will darken the leather and will smooth out the roughness such that, over time, it will look more like smooth leather. Pay particular attention to joins, seams and welts. Use several thin layers, allowing say a day between each application to allow the leather to draw it in. It is best if you do this in the warm - the warmer it is, the quicker it will absorb. Some even warm the tin to liquefy the contents and apply with a brush. I use a hair dryer to speed up the wait between coats. My own roughouts at the end of eight or nine years (which were also polished with brown polish from time to time) took on a good shine and not 'rough' at all. Looked used but smart (and more importantly were waterproof!).
Note that, rather oddly, while I don't have any evidence to back it up my gut feeling is that dubbing boots makes them colder to wear - whether it makes foot heat easier to escape of for cold air to transfer to the feet I dunno, but they definitely feel colder!
As for which product to choose, well it matters little. I like Chelsea Leatherfood (made in the UK) which is light and beeswaxy and quick to absorb. But I also use Cherry Blossom (made in the UK) as well as anything else that comes my way (some 50 or 100 years old!). You can get big tubs of 'Gold' dubbin on eBay that works out very cheap (but I haven't tried that stuff).
(Oh, and shameless plug for a dubbin website 
http://www.blancoandbull.com/boot-cleaning/dubbin/ )
there's no shame in plugging dubbin,i used it all through the cubs ,scouts and venture scouts.i get a great deal of satisfaction from proofing boots,im a great advocate of useing you'r fingers as they warm and melt the stuff so is soaks in nicely, plus you always have you'r fingers with you or they can be easily found unlike trying to find a hairdryer in this house.polish after proofing if thats the look you want.
armoury
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Yes, with fingers works and was advocated on early tins (although I prefer a more refined approach ). Hell, it was even used as a winter hand protective as well "A relief to chapped hands").
Yes, with fingers works and was advocated on early tins (although I prefer a more refined approach
Remind me never to shake your hand in winter.
Another option is to melt the dubbin then use a stiff brush to apply, this allows it to be worked into all the nooks and crannies and being liquid is absorbed very quickly.
(Oh, and shameless plug for a dubbin website
*M&S Advert Mode* This isn't any dubbin website, this is a Chomley-Warner dubbin website... *M&S Advert Mode*
Speaking of adverts, I've got some original, unused tins of ww2-era dubbin, if anyone's interested. £5 a pop. 
FWIW, here is what I did to my German low boots which are partly 'rough out'.
http://www.ww2airsoft.org.uk/php/phpBB2/viewtopic.php?f=45&t=12171&p=186698#p186213
