Easiest way to write a grammar?
Of course "instinct"always helps, but writing a grammar is more or less writing a program, but with limited resources.

Your example matches a regular language. Those languages can be defined using right-linear grammars but I prefer finite state automata. Programming linear grammars (and finite state automata) requires thinking in states. What is the characteristic information about the string I have seen so far.

In your example the "state" consists of the last letter read. OK, since your task is to avoid reading a letter twice. In other examples you might want to do some counting ("if the string contains exactly four $a$'s it ends in $b$") or modulo counting ("the number of $a$'s is even"). Perhaps you want to do a kind of pattern recognition ("words that do not contain $00110$") and you keep track of the part of the pattern you have just seen. 

Sometimes the requirements consist of a Boolean combination of properties. Then you keep track of both of them in the state and react on what you know. 

You see: just like programming.

If the grammar is more complicated, like context-free you probably want to think in terms of trees. That is the structure they use to derive strings.