Before accepting any type of deal, be sure that the product in question legitimately fits in with the movie. A movie set in the year 2402 would not fit with a laundry detergent product.
If the product does fit with the movie, find out exactly what the company wants to get across. It's one thing if Jennifer Aniston uses a specific laundry detergent, but it's another if she speaks wonders about how it got a specific stain out that no other detergent could get.
Once the goal is established, pinpoint 1 to 3 instances within the script wherein the product placement could occur. One to three should suffice, as you do not want to overdo anything.
Alter the script slightly in these 1 to 3 instances so that the product placement makes sense.
The script alterations can even occur before the scenes in question. For instance, the character of Aniston's son can be seen playing football early on, and nothing gets stains out. Well, now you can show her using the new detergent and the results working.
Regardless of Steps 4 and 5, do not make any alterations that come off as forced or unrealistic. This ties back to Step 1, making sure that the product fits the film.
Understand that the best product placement is one that nobody even grasps is product placement. If it sticks out, then the idea needs to be much more subtle.