Get away from the busy backpacking centres! Although this may sound counter-intuitive, it is sometimes a lot easier to meet people when there are less of them about. You can get awful lonely in a crowd, especially if you're not exactly dripping with confidence. If you meet someone in a far-flung, semi-deserted spot, then chances are that the unlikelihood of your being there in the first place should be enough of a conversation starter to be getting on with.
Conversely, you may find that if you try and strike up a conversation with a group of backpackers near a busy hostel or popular tourist trap, you stand a greater chance of being given the brush-off. Mind you, if you're going to be starting a lot of conversations with strangers, you'd better get used to that anyway! If you're of a particularly outgoing nature then it probably doesn't matter, but if you're shy and looking to build confidence, you'll find it easier to do that off the beaten track.
If you're out for a bite to eat on your own, and spot another lonely diner, why not ask them to join you? If someone is looking around a lot, or reading a book, then there is a good chance that they are indeed dining alone, and might relish the chance to find some company. Be prepared for the odd brush-off of course, but don't let this damage your confidence - where one person will decline your company, another will welcome it! Getting to deal with some minor rejection will do you the world of good, and you'll find your confidence will grow in the long run as a result.
Wear something or carry something that might help you strike up a conversation starter, such as a hat that advertises a particular country, or a t shirt that signals one of your interests. This may go against the grain of any notions of cool that you might have, but when you're away from home they can invite conversations with fellow travellers and locals with similar interests to you.
Choosing a shared hostel, or any other shared accommodation is the most straightforward way to mix with your fellow backpackers. By opting for a dorm room you'll inevitably end up having a chat or two with your roommates. Even if you can't find accommodation of this type, most hostels and guesthouses have communal areas where you can meet your fellow residents.
Common sense it may be, but you'd be surprised how many people forget how important it is to simply smile when you want to meet new people. A more positive outward appearance will attract others. So, even if the road is starting to get you down, put on your happy face, and you'll be far more likely to meet people and have fun! If you're looking sad and lonely, you stand a much greater chance of ending up that way.
Be helpful. If you see that someone is lost or is in trouble, help them out. This is a great conversation starter and helps to tighten the bond between travellers.