Before going to a packaging designer, you need to have a understanding of what sort of package you are after.
The packaging designer will ask a number of questions based around what you have in mind for your product so it would be good to have some ideas in mind before hand.
An option would be to have a look at product packaging that is similar to yours normally this would be your competitors packaging. This should give you a feel for how the same/similar products are packaged currently. Armed with this information you can then get ideas for any improvements that can be made to the existing design to make yours stand out and get noticed.
Have a think about the size of the package and how much packaging material would be required.
Have a think about the material the package would be made of (card, paper, plastic or eco friendly products).
Have a think about the colour scheme of the package and if it relates to the product inside. Also how the colour would affect the sale of the product.
Eco Friendly Packaging
Eco Friendly packaging is starting to become more and more popular now and can even be a USP for the product. Depending on your product it could be a good option to opt for something that is either recyclable or is made from a recycled product. This could also link into a good PR campaign for your product/package. PR agencies do like to use stories such as this in generating you media coverage (consider Innocent Smoothies for example).
Layout Of The Packaging
This is something that the packaging designer will help you with, but care needs to be taken with regards to how much of package is given over to the design and how much is given to the essentials (ingredients, address, trademarks etc.)
Budget For Your Packaging
Have a think about what sort of budget you want to apply to the design and production of your package. Having this in mind will help you when selecting a suitable packaging designer for the job of putting the ideas from paper to reality.
Once you have done all the above, you are equipped to approach different packaging designers and talk through options about getting designs together for your package.