Mr Goodman is on the right track by suggesting you concentrate on what you already sell the most of.

I want to extend this to the other side of that equation: using 80-20 who are your best customers? After all, online you have practically unlimited customers and that's where the sharpest advertising money starts it's day.

So take your best customer and speak to them directly. Write copy or tweets or facebook comments as if you were speaking directly with that best client. It'll take some imagination, it does work. Have a look at this site - they've taken their best customer and gives them what they want. http://www.jpeterman.com/Womens-Skirts/Trevi-Fountain-Skirt

The point is that when you say "If I look at big brands they advertise one product properly for many month to really get their story across" taking the approach that speaks to your best customers you'll only have to do it the once.

So how can you do this with something as pedestrian as horsefeed? It's not that hard, really. Look at what motivates your customer to buy your feeds and focus all your advertising on that. Plus write about the results it gives. Try three different angles (= three different motivations) and you'll find one that comes out tops in terms of online advertising. Hone this for all its worth and you'll have an advertisement that speaks directly to your best customers.

Having started with that, you can broaden this approach to your other products.

Do you have a newsletter? Keeping people in the loop is key in our modern world.

DOUBLING SPEND - firstly organize your campaign so that it is working. THEN start spending more - because it's like putting a dollar in and getting two out. Get this right and the spending questions answer themselves. Online advertising - PPC/Display - are so reflexive that you can try practically any approach and see if it works. You'll discover things about your clients and their approach to your products that you can use across media that are way less easy to judge the effectiveness of.

SO: start with your best products. Worry about the poor performers later. What's more there will be good reason for them not being good performers - however the fact that you put one in the order with twenty of the others means you'll sell those twenty. My local supermarket cut back on my favourite soy sauce - so when I need some I shop elsewhere. I have to. The supermarket saves a tiny amount of shelf space and loses a whole basket of profit. If you work out what products support others with an analysis of orders you'll be able to work out a strategy to get them in their proper place.

It could be that you've dropped a seemingly poor performing product that was driving bigger sales of other things.

What do you think?

All great advice...you may want to try a 'halo' approach in each market silo. Featuring the best/most popular product (telling the story or reinforcing same)...the 'halo effect' is using the feature item as a lead to another product.
If done properly, the 'secondary' product, being haloed will connect with your customer as a equal or close to the superior product

Firstly, as regards your three strategies, I would set up a PPC campaign for all your products - after all, if the ads aren't clicked on much, they won't cost much, will they? Concentrate on your best ones first, get the data and use that info (leverage) to promote your lesser stuff more effectively.

You say "However, it is particularly hard to measure the magazine performance." - that's the beauty of a display network campaign. You can target magazines online, get the data, refine everything and know that when your ads hit the printed versions they'll be as good as they possibly can be. What's more you'll know which magazines to target too! Remember that the direct mail guys only had magazines and newspapers - and it took thousands of dollars and months and months of work to get results. That did mean they had to be really, really sharp.

As to copy, you need to hone this to your best clients; you can discover this in various ways. The best is to speak to them directly.

Finally: take a look through your sales book. There will be products that sell together in groups. Like flower seeds are always sold with a few tomatoes as well, rarely with other vegetables. Vegetable seeds are sold with Nasturtiums and Marigold seeds - rarely others. So take some time and look out for any patterns that develop. That's when you can use CookMarketing's ideas to the max. In an online campaign you can have the "others bought this". If you have the time, you can set this up on your website and get the data that way - I am no analytics expert, my job is very much front of house. What I can do is get the analytics guys working properly rather than puzzling out their own crazy solutions.