Vincent, have you seen the book "Basic Polish: A Grammar and Workbook" by Dana Bielece. It's not cheap (per page) but it is pretty good... Bite-sized lessons and exercises.
If it's as good as her other Polish grammar book I'd second this recommendation.
Vincent, glad I could help. I know studying the declensions gave me a few headaches when I started learning Polish. Complicated but absolutely essential.
Instrumental (narzędnik) is the easiest Polish case in terms of rules..
Indeed. I don't think I could have summarised the suffixes for other cases quite as concisely.