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.