Honk if You Want to Register Your Bumper Sticker as a Trademark!

Dear Rich: I am in the midst of applying for a trademark and am confused about which category in the trademark manual my item falls.  The item is essentially a slogan (code 045) which is being printed onto bumper stickers (no code found) and also onto baseball caps and hats (code 025). The intent is to expand the list of goods sold that are branded with the slogan. The bumper stickers and caps are the two initial items I will be selling. Which category I should enter on the application? We think you'll have a hard time registering a slogan with the USPTO unless you can demonstrate that the slogan is associated with a product or service -- that is, a product other than caps or bumper stickers. For example, a slogan like the one shown above would be difficult to register, as would variants such as

• Honk if You Like Cheeses, 
• Honk if You Would Like to See My Finger, 
• Honk if You're Going to Run Me Over, 
• Honk if you Like Prune Tacos (our favorite), 
• Honk if You Like Peace and Quiet, 
• Honk if You Are Living a Life of Quiet Desperation, or 
• Honk if You've Never Seen a Gun Fired From a Moving Vehicle (scary). 

That's because these slogans are considered ornamental (they inspire, entertain, or amuse) or informational (they express a thought). To acquire registration they must create a consumer association with a product or service (for example, Honk if You Sell Car Horns for an applicant that sells car horns).
Honk If You Receive an Objection from A Trademark Examiner. Upon receiving an application for  a slogan, the trademark examiner will probably object on the basis of § 1202.03 or § 1202.04 of Trademark Manual of Examining Procedure or TMEP (excerpted below).

§1202.03 Refusal on Basis of OrnamentationSubject matter that is merely a decorative feature does not identify and distinguish the applicant’s goods and, thus, does not function as a trademark. A decorative feature may include words, designs, slogans, or trade dress. This matter should be refused registration because it is merely ornamentation and, therefore, does not function as a trademark ...
§1202.04 Informational MatterSlogans and other terms that are considered to be merely informational in nature, or to be common laudatory phrases or statements that would ordinarily be used in business or in the particular trade or industry, are not registrable ... 

It's on these basis, for example, that the slogan THINK GREEN was rejected as a trademark because it was merely a statement of environmental awareness. The slogan, HAIR COLOR SO NATURAL ONLY HER HAIRDRESSER KNOWS FOR SURE was registered because consumers associated the slogan with a particular product. The only way to overcome this objection is to show that consumers associate the slogan with your products or services.
Honk if You've Used the Wrong Classification Codes ... Also, you referenced code 045 in your question. Class 045 is for legal services (Honk If You Like Litigation) so we don't assume that's what you meant unless you've come up with a slogan for a law firm (Honk If You Like to Pay For Overpriced Associates).
Honk if You've Tried Copyright Protection. Finally, a bumper sticker slogan cannot be protected under copyright law as copyright does not protect short phrases. A short phrase can be protected in conjunction with an illustration or it may be protected in some cases, if it is taken from a larger well-known work, such as taking a line from a movie.