作 and 做 are both translated as "do" or "make"; how can I tell when I should use one or the other?
Some real life examples to help identify the differences:
In general, "做" is almost always a verb that has some real meanings. It is very close to English word "do" in its verb form and "make". E.g. "Do your job!" = "做好你的工作!", "do nothing"="什么也不做", "make love" = "做爱", "make some cakes" = "做些蛋糕", etc.
For the word "作", in my personal opinion for now, acts as a function word (虚词) more often than having some real meanings. E.g. "作为" = "as/act as", "装作" = "pretend as". In these examples, it is really close to "as".
One of the most common "real meaning" usages of "作" is "compose": "作曲" = "compose a music", "创作" = "compose/create/write".
Another very common exception of "作" is the word "作文". It is a noun means "essay" or generally the "writing assignments" for students. Without checking it up, I don't even know what exactly "作" in "作文" means.
All in all, "做" is much more widely used as it is really similar as English "make" and "do".
Flake has a really good answer, but I thought I'd add a favorite example of mine.
工作 and 做工
Carving a spoon out of a piece of wood, or creating a vase out of clay is 做工. A highly tangible result of what you do with your hands.
Getting up in the morning and going to your 工作 could very well mean doing some 做工, but whereas one is high level and abstract, the other one is quite specific (and often relates to working with your hands).
These two differences apply to the usage of 作 and 做 in other contexts as well.
做ing a vase out of clay means actually shaping a lump of dirt into somehing that holds flowers in a pretty way.
I could be totally off-base on this, because my Chinese experience is colloquial Cantonese in Hong Kong, but here's my $0.02. Generally from my experience, 作 is used in situations where you're doing something that has an end and an output, such as the Cantonese phrase 作准备. 做 seems to be used for things that are more ongoing, such as 做工作. Please somebody come along and correct me if this is wrong, because I'd like to know the answer to this as well.