Seeking best practice for maintenance billing

Since the first day with netsuite we have struggled with properly accounting for and billing our recurring maintenance & support plans.  We have 3 plans - Bronze, Silver and Gold - each with a pretty standard "% of sale" as the annual cost of the plan.

The problem we are running into can be summarized in three areas:

1) How do you set up netsuite so that you can easily, through a simple search, identify customers who need to be billed for their upcoming M&S renewals?

2) Given #1, how do you account for changes to their M&S charges during the year - i.e "customer x buys M&S on product 1, then six months later buys M&S on product 2".  In this example the customer would need to be billed a full year for product 1, then a pro-rated amount for the remainder of the year for product 2 BUT would need to be billed in full for one year for products 1 AND 2 on their anniversary date.