Cooking beef: how to make it tender?
The cut is important for both techniques. For sauteing, you need a lean cut - fillet, sirloin, or good rump steak. These should be cooked quickly over a high heat. As Cerberus has suggested, if you are cooking something else in the same pan, take the beef out and re-add it later; don't boil it in a sauce.
Stewing beef needs some fat and cartilage which breaks down during slow cooking and tenderises the meat. Packs of such beef are usually sold as such in the supermarket; look for a pack with plenty of fat marbled through the meat; 'Lean stewing steak' is as useful as a waterproof teabag! My personal favourite stewing beef is brisket, as it breaks down into nice tender fibres after 2-3 hours cooking.
Contrary to popular belief, browning meat does nothing to 'seal in the juices'. It simply provides a bit of extra colour and flavour by 'caramelising' the outside a little. So if you have a strong flavoured stew (like a chilli, for example), just throw the chunks of meat straight into the sauce.
Browning beef may not seal in the juices, but the Maillard reaction is still valuable for taste. One should brown the beef in advance for the extra taste.