What I found in dealing with agencies was three prong.
One prong was to talk to the 'media planner' about what promotions and sales were coming up. This way you could educate the rep on the market and how the local store competes. After this, knowing the promotions allows you the opportunity to create unique proposals that will look good when the decisions are made. Always asking what they want is key. What I'm really saying is you actually talk to the ad agency and help them do the best job they can. It is amazing how many radio people see ad agencies as their enemies. Many stations think they'd have the dollars if the agency wasn't involved.
Second is involving the local store(s) manager. When they are fighting the fight along side you, you have a better chance. Sell them on any idea you have. Suggest they call the ad agency or ask their supervisor if he/she can influence an advertising decision. Trust me, if a store manager calls the ad agency they listen; same goes with supervisors. They know managers know their stores, the market and have a reason (financial or bragging rights) to do well.
Third is simply getting on the buy list. This involves getting the station info to them and tailoring it to their specific clients. It never hurts to send, say an insulated large glass with your station call letters that might be filled with a soft drink or iced tea, coffee cup and simply anything with your call letters on it. If the rep will use it, you're always top of mind. When I was on the border working in radio, I'd send, say, a 2,000 Peso bill (back when that might cost about a dollar) with a note saying "Peso little, get so much". These little gifts work wonders. Frequently when you get on the buy list, it is months before you get an order and once you're on, you typically don't go off the list. Never discount the value of a real relationship with the agencies. Putting a face to the voice is like a miracle. Regular contact via little gifts or a call works wonders.
Building promotions is very important if you'll beat their cost point. I recall one year a cafeteria was going to do a campaign for Valentine's Day. I worked a proposal where I tied a flower shop, musicians, jewelry store and a couple of other businesses to a promotion for the cafeteria. The value of everything was about 3 times the dollar value I asked for which was the TOTAL budget for the market. I asked for exclusivity and got it. We let people ask their special someone to marry them. The future partner called in within the allotted time to accept and one of the couples got an engagement ring from the jeweler; all others got a substantial discount. We had live music at the cafeteria and ladies had a flower (mostly carnations) pinned on them when they entered the restaurant. There was more to this (ie: each diner got a value equal to their meal, if not more, in freebies), but all said and done, I took businesses who I would not gain a schedule from and tied them to the cafeteria and won 100% of the budget, not just for radio but all media in the market. To today's dollars that was about $6,300 when our typical spot rate was $8 in today's dollars.
Let me conclude by saying most folks at the agencies love personal contact since it is rare. They love knowing about your market since they're typically flying blind and trying to do the right thing. They love anything you present to this that tells their boss they got the best value for the client. Thus, go over the top, get in touch, stay in touch and go all out to make the agency look good while becoming the radio person your rep can count on to be a friend.
Don't underestimate anger as a reason. Yes, anger. I went to a nationally known donut store owned by a franchisee. He was kicking in 6% of his gross to marketing and all the dollars were being spent 150 miles away in the big city while he got no local dollars. He quit paying his 6% until he had local advertising and he got it: a good constant campaign with us and weekly inserts in the Sunday paper. After he refused to send one payment, and didn't, their attorneys threatened and he said to bring it on. It was less than 2 weeks later the buy came in. At one clothing store the new manager found out his bonus hinged on beating every store in his district which included the big city. When the agency said they'd spend nothing in a market with a stand-alone store, he was furious, called his supervisor on the spot and by mid-afternoon the agency placed a buy. Seems he was one of their better managers who had been sent to a struggling store to get it back to its previous success. The supervisor was so upset he called me to ask me to call him first because he didn't want to make the manager angry.
Have fun. When we tried to get Kinney Shoes on, they were using 'always the right shoe' as their line. We went to the store, bought a close-out pair of shoes and sent the right foot shoe to the rep with our media kit saying we were 'always the right buy'. That got us noticed. Even some brochures from the Chamber of Commerce is nice. Let them know as much as they want about where your station is located. At stations on the border where I worked we sent little miniature bottles of Tequila to reps with some other little souvenirs you'd find at the tourist shops. The idea is to stand out.