Mmm it's a tricky one. The ideal solution would be to create a detailed questionnaire for her to fill in every time she gives you a new job covering everything you will need (which depends on what sort of copywriting you tend to do and what your business is). You would have to craft this questionnaire carefully so that everything you might need is covered.
As she is very busy she may hate this idea. However, you could present it to her in a way that highlights the positives of filling in the questionnaire, for example;
'i would complete the project more quickly'
'i would not be continually asking you questions throughout the project'
'it would help me to produce even better quality work'
If she agrees you would have to get this process formalised - i.e. get her to agree in writing to provide you with a written brief in the form of a questionnaire.
From what you've said though I am not sure she would go for this!
