Hassle, in the way of pugnacious touts, salesmen, rickshaw drivers and beggars in most less developed countries, starts the moment you step off the plane and only stops when you go to sleep or step back on the plane. It is at its worst where there are high concentrations of tourists, in the Asian Sub-Continent (worst: Northern tourist circuit India) and North Africa (worst: parts of Egypt and Morocco).
The most unpleasant thing about hassle is that it makes you jaded to locals and compromises your appreciation of some beautiful places/people. Try enjoying a walk down the Nile in Luxor at sunset or a appreciating the spenders of Agra/Delhi in India, the latter being places where all travellers seems to be permanently on edge! Some of the worst places in the world are the tourist hotspots of Egypt, Morocco, India (particularity the state of Uttar Pradesh and Rajasthan) and Indonesia (notably Kuta beach in Bali). However it is worth noting that the following text and advice does really relate to these kind of hotspots and is not reflective of the vast majority of the world including the Americas where hassle is limited.
Humor goes a long way and keeps you and everyone in a good mood. Street seller approach you selling an item that even they, for sure, know you won't want. A guy offers a huge bar of laundry soap - smile and say 'no thanks, no water'! Rickshaw driver insists you visit a shop, let him know you are desperate for the toilet and if you don't get to your destination fast you'll soil his vehicle! Taxi driver repeatedly offers his services, tell him you're on trip to walk across the country (in India suggesting you're on a Salt March is a good one). You get the picture. One suggestion is to ask 'is it free?' in the local language, which works a treat. After this for the more persistent breed who may still be bothering you, be firm, make eye contact and let them know your answer clearly and politely, and then ignore. If you keep saying no, looking at what is on offer or didn't make it clear the first-time, it sends the message you are not sure.
Simply ignoring is often the best policy as it can be difficult to respond to everyone particular when you are greeted which volley of hassle you might find around major monuments. Just keep taking to a companion and/or walking. Often responding in anyway can and does encourage hassle, likewise asking for a price of an item you are not serious about buying will give you increased hassle - you'll often see touts/salesmen/beggars following tourists for great lengths of time, because they smell a commission/sale/donation in their actions. It's not pleasant being followed, but it's one of many physiological tactics you'll find employed, all of which bully you to giving in to what in real terms isn't a huge amount, but quite simply the golden rule is don't get bullied. If a driver or other asks for a tip or a greater than agree price for his service and you feel he doesn't deserve it - don't give it. If someone offers to help you out of kindness and then asks for payment or someone takes payment and keeps standing with his hand out looking disrespecting at the agree amount you gave, don't fold - chances are it will only be over a small amount, but by giving in you increase hassle for the next traveller and that small amount would be better given to a needy beggar who isn't targeting tourists. Don't be made to feel bad about it, if you are clear at the onset you are not in the wrong.
The same goes for operators (be them hotel or tour) and especially rickshaw drivers who hack up a price or maintain they misunderstood your agreement. Again be firm and don't get bullied. If necessary leave the agreed money on the rickshaw seat (they often will not take it by hand) and just walk off. You soon learn to be firm and very clear in agreeing prices/services. Make sure you are understood and do not give into any, 'yes, yes' or 'as you like' agreements or acknowledgements. In places like India, with the right (smiley, not ultra-serious) attitude and use of eye contact, you will get a lot less hassle after a few weeks.
Equally drivers, salesmen and strangers will often bombard you with question like 'where are you staying?', 'have you visited this or that?', 'where are you from?', or anything that gets you saying yes. On the whole once they get talking, on friendly terms and have enough information, services offered are much harder to turn down. Once again it's just another tactic and as always there is rarely a need to be rude, but if you don't want something say no and if at the first question motives are clear, don't enter into the conversation. A variance of this and probably the most jading and invidious instances are when you meet a nice guy who after taking the time to talk to you and often helping you, turns out to have alternative motives and is selling something (see our page on Confidence Tricks).
To summarize, you will soon develop your own techniques - be firm, polite and accompany your firm 'no thank you' (in the local dialect if you can) with a smile. After that don't make eye contact, don't keep repeatedly saying 'no' or get angry. With salesmen, if you show any more interest this is when touts are particularly determined and most frustrating.
On some occasions beggars, especially children, will make body contact, tugging on your clothing. In this case remove their hand and looking them directly in the eye, make your 'no' clear. When a beggar or salesman sees you have no interest, they soon move on to their next target. Remember constantly turning around to say no over and over again shows you are obviously not sure and worthy of further hassle.