Click your affiliate links on the offer page in the affiliate network. Then save the merchant's sales pages to your computer as a web archive. It's another critical step, so don't leave it out. It will help with your landing page design, advertising style, and serve as evidence if you need it later on.
The merchant's sales page is where the conversion takes place. You'll want to mirror the tone and language on the page in your PPC ads, display ads and landing pages. The closer they are in alignment, the higher the conversion rate will be.
Use the Digital Color Meter app (it comes with the Mac) or similar software to record the colors on the merchant pages. You can use the same color harmony on your landing pages and display ads, so they look similar to the merchant.
Note the photos on the merchant's page. There's a good chance that they are stock photos. You can use a stock photo service to find similar photos, or the exact same photo as they use. If you use similar photos, portraying similar emotions, or situations, the conversion rates go up.
If the offer has no restrictions whatsoever, you could use Snagit to copy their logos, product shots, etc, and use them in your advertising, so long as you are selling their product in a fair and ethical manner. Some companies even provide a "press kit" with logos, product shots, ads and writeups that you can use without needing permission.
Then there's the dark side. Every once in a while a merchant tries to pull a fast one.
For example, I was running a credit card offer from an online loan company. The offer was converting for a few months then suddenly stopped. The merchant, without telling anyone, took down the credit card page and put up a loan application page in its place. Commissions stopped but they were getting free PPC traffic to the loan application from every affiliate running the offer.
In this case, you will need to show your affiliate manager at the affiliate network the original landing pages, because they don't usually keep a record of them. Having good evidence of the original offer makes sure the bad merchant gets booted out and you still get paid.
Laziness may appear attractive, but work gives satisfaction.