Toward Better Tones in Natural Speech - Dr. Liao (of the Defense Language Institute) suggests re-conceptualizing tones in Mandarin to take into account natural speech patterns and tone changes within sentences:

Pronunciation (which tones are stressed) will often change depending on previously spoken words. I’m not sure there’s a good way to study these except memorizing lots of sentences. 

For example, if you’re coming off a second tone, it’s very easy to say a first tone or third tone, but much harder to say another second tone – which is why the first instance is often stressed less. For example, say 美籍华人 ten times fast – this can make for an intense oral workout. I wonder if anyone is marketing Mandarin courses by stressing the beneficial aspects of regular mouth exercises?