The secret to a good paella, as with risotto, is to use a good stock. Whether it be chicken, fish or vegetable, the quality of the stock is paramount as the flavour is absorbed into the rice. I try to make my own stock if possible, but obviously this isn't always practical - I find 'Just Bouillon' stock cubes about the best readily available ones.
My recipe sweats plenty of onions in olive oil, then add garlic, and the rice. Let the rice fry gently for a minute until it looks glazed, and add a good glug of white wine, then the stock (I put a good pinch of saffron in the boiling stock, and leave it to infuse for 5 minutes before using). Any thing else you add is up to you... fresh peas towards the end are nice, and most seafood (prawns, cockles, mussels, fish) should be added towards the end as it requires minimal cooking. Obviously, if you use chicken, it should be added before the stock. Once the majority of the liquid has been absorbed, I like to finish the paella with fresh chopped tomatoes, and chopped parsley.
Wow, I've just realised i've written an essay...sorry! :os