SALES: It varies by the size of the company.   Small companies generally are hired because they are local and cheap for small projects. Small projects get more attention and quality from small local companies. Big companies hate small stuff and are expensive. Selling to local companies is really just a matter of knocking on doors and showing work until they give you some of it. Large agencies are generally hired for their breadth of services, ability to scale, and strategic understanding.  Large agencies are able to attract and pay for a lot of talent in sales and delivery.

Most opportunities are found through relationships between people who know each other.  But customers are always seeking new agencies and ideas. So customers will sometimes seek out an agency that wins awards or does promotional work for interesting clients.

But most new companies do not have relationships and must generally produce gratis work for non-profits to promote its abilities. Much of the best award winning creative work is done gratis.  Usually, established companies are too conservative to fund projects that are useful for the agency to use in a sales pitch.

Rarely do companies get off the ground without one or two accounts to support the startup.

If I understand your question above, ‘Design Services’ is what you’re selling. 
The problem is that for marketers, design services are like buying paper towels, toilet paper, and dish soap: they’re commodities. Design isn’t scarce. The difference between all but the top talent is marginal. So to get clients, you need to sell something other than the work itself.  Generally, you’re willingness to do it cheaply, or with greater customer service. Or perhaps because you understand their business or customers.  Largely; it’s “ease, dependability and price”.

Most agencies MARKET rather than sell themselves.  Most service companies SELL themselves rather than MARKET themselves.  The question is, whether you have the money and talent to market yourself, or whether you are still just a service company and need to sell commodity services directly until you have relationships and business understanding. 

PROCESS: 1)if you’re small just knock on doors and learn about possible client’s businesses.  Eat whatever ‘bugs’ you have to in order to get in the door.
2) Develop a pitch team of Creative, Editorial, Technical, Marketing and account management.  Most of the time, in my experience, there are only two strong people out of that set in any given company. 
3) if you get big enough, then hire a salesperson.  Usually the founders of small firms perform sales.   Sales people are very risky. Almost all business I have purchased in my life have gotten in trouble when the founders try to stop selling and hire salespeople.

RFP’s: have a very bad name largely because customers will steal ideas, and because most of the time you’re just ‘column fodder’.  Pitches are EXPENSIVE.    A big agency for example only might put in five pitches a year. But they would spend hundreds of thousands of dollars on each pitch. A midsized agency might spend over 50K for each pitch and do more of them, and a small agency less than that. It’s expensive.  A commodity agency might never pitch just sell services based upon proposals.  So, if you’re in the pitch business, it’s best to pick the RFP’s you’re capable of winning and then to absolutely kill it with good ideas, and price on the pitch. 

My main bit of criticism, as someone who almost never loses a pitch, is that it’s not worth pitching something that you havent given your all.  So only pitch when you’re willing to give it your all, and where what you’re pitching is really valuable to the customer.  Everything else is a waste of time and money.  Count on at least one-quarter of your business leaving each year, so that if you want a greater than 20% growth rate – which is what attracts customers and talent – then you need to sell enough pitches to generate 40-50% of your revenue a year. If you figure out the average size of your accounts as they exist today, then the size of the pitches you feel you can win, then the rate of your wins, it’s just some simple math.   (Most agencies are puny, at under 5M in billings.) 

(I’m trying to keep this simple enough for a Quora posting, so if something isn’t clear then ask.)

SOFTWARE: Adobe suite. Macs. You need to be able to speak PC well enough to work with and deliver customers assets though.

REFERENCES: There are notoriously few books on this business that are worth reading.  Ogilvy on Advertising is about all you really need to know.  There is one on copywriting the name and author escapes me.  Maister’s book on being a “Trusted Advisor” is as timeless as Ogilvy’s.   Other than Seth Godin’s attempt to shock the old guard into thinking about the identity of consumers today little has been written that’s really valuable. (There book about the marketing history of Mazda is good too.) Generally, high minded and fashionable books on marketing and advertising are just nonsense.  Find work. Take care of clients. Accumulate talent.  Try to survive. It’s a craft. Not a science.  It’s not that complicated.

LAST BIT OF ADVICE: Creativity is not magic. It is the process of filling your mind with related information then playing while the subconscious does its thing.  It’s repeatable. It’s procedural. And you can get good at it as an individual or team.  The best defense against doing bad work is to simply collect as much work as possible and keep examples of both good and ‘failures’.  I can’t tell you how many ideas I’ve shot down by using an example of a known failure.

OVERALL: It is a murderously overpopulated business in transition from a highly profitable past to a less profitable future, where you are little more than a commodity and where you live hand-to mouth in exchange for the freedom to work in a field that accepts “playing” as doing work.

https://www.quora.com/How-do-the-best-graphic-and-web-design-firms-handle-sales