1) Plan your manpower to ensure people are ready to move into new roles, or ready to take on promotions. By doing this you can ensure that you have the best, trained and experienced people in the higher roles and you are recruiting at the lower levels and training people to move through your business. Recruiting at the lower levels tends to have lower costs. 'Home grown' staff, trained in house, are often the best placed to take on higher level roles in your organisation. That's not to say there is no benefit to bringing in new blood, there is, but be prepared for this by planning your resources and staff moves in advance.
2) Hold open days at your business to promote your organisation to both customers and prospective employees. These are fairly low cost and can net you a good haul of applicants. To ensure people do know they can apply for roles post the roles on your website along with the details of the open day. Have application forms available and allow people to talk directly to the recruiting managers. While the days themselves will use a lot of manpower they can cut down on time spent recruiting overall. If you were very organised it may be possible to arrange on the spot mini interviews to short list candidates. The day can also be advertised, usually free or at very low cost, at local universities, colleges and, of course, job centres. A simple ad on Gumtree is very low cost, and Total Jobs (and similar) sites will also carry such advertisements - introductory rates of under £100 are available for most recruitment websites.
3) Work closely with your recruitment agencies, keep them close and keep them in touch with your business, even when you are not recruiting. Agree fixed fees with them as opposed to rates based on salaries, make them competitive by having a Preferred Supplier List and reviewing it regularly. If you are recruiting in bulk then the agency could do the work for you, cutting your in-house costs - ensure they fully understand the job and person spec and the business and ask them to shortlist CVs/applications for you. Once you have the shortlisted CVs only a quick sift is needed from HR before they are passed to the recruiting manager. Recruitment is a very competitive market at the moment so agencies will be open to conversations about fees, especially for long term relationships and large scale recruitment campaigns.