I want to begin by saying, "I really feel sorry for the company for losing trained resources to it's competitors."
Having said this I quickly wish to add that the problem that you are in is not a new phenomenon it's been there for ages. And each of the companies that were hurt by this "poaching" act, devised means and methods to counter this menace and succeeded in their own way. That may not have happened unless they did some serious introspection and thinking to generate the right strategies to stem the rot if not eliminate.
The point that I am trying to make is that there can be several reasons that one can attribute to this happening with respect to your company. If I might say, "you perhaps choose potential people and give them good initial induction training to make them productive," then that seems to be the one that has led to your competitor's being attracted to hire people who choose to leave your company.
That apart there may be something that is missing in your company - it could be the management or the policy that precipitates acts of exit.
There are several reasons cited already here as apart of the discussion and the theory world over would have these and they would have added a few more.
This is valid, and can be true, which I may choose to repeat
Unhappiness with the "Boss"
Inadequate compensation
Nature of the work itself
Shifts
Recognize outstanding work
Offer professional development opportunities
etc.,
Years ago, I read this on the Internet, the source of which I am not sure, and so I'd acknowledge Source - Anon
I'd like to narrate a story which highlights the point the original write was trying to make
A frail old lady was waiting at a bus-stop with load of bags hanging on her shoulders apart from the one's in her hands.
After along wait a bus came that way and stopped to pick the waiting passengers. As soon as the bus stopped, young students, rushed into it and as the old lady was struggling to make her way on to a vacant seat in the end row, she got pushed and she fell down.
Reacting to this incident were some of the passengers, " How rude, these days young kids are, didn't their parents teach them any manners," yelled a lady who was a teacher.
"Such people should be booked under law and be punished, " yelled a man, who was a lawyer.
"Oh! No, she would have probably broken her shoulder bones, she needs immediate medical attention, " was a shrill voice who happened to be a doctor.
Like these each one was passing his/her comment but no one really went close to the lady to help her get to her feet and then to her seat, nor inquire her about how she is feeling till one person actually did this, by helping her stand and then made her sit on the vacant seat, and he turned out to be a Welfare Officer in a local manufacturing company.
This story clearly narrates how people's thinking and behaviors shows up according to their professions - the teacher, the lawyer, the doctor, and the welfare officer acted or reacted to help a frail old lady crying in pain. The passengers forgot the fact that they are human being first and then professionals. This may just seem like a story, but professionals get greatly influenced in their thinking and behavior by the profession which they are in.
Keeping the HR part of it aside for the time being I would want to suggest that you try creating an "Employee Branding Exercise"
It'll be useful for you all as the company's management team to ask the following question and to get answers for that which will define the roadmap for brand builidng.
SWOT Analysis
• What are your strengths as an employer? Your weaknesses?
• How does top talent rate you against your competitors?
• What do your competitors offer that you do not?
• What do you offer that your competitors do not?
• What do employees like about coming to work for you?
• What do employees dislike about coming to work for you?
• What drives job seekers to apply to your jobs?
• What prevents job seekers from applying to your jobs?
• Are you offering competitive pay?
• How could you better position yourself as an employer of choice?
• Where should you be advertising your jobs?
After this done, and all of you have a consensus findings, plan a "branding exercise
1. Defining Your Message
2. Communicating Your Brand
3. Evaluating Your Message’s Effectiveness
4. Assessing Your Employment Brand
This is slightly time-consuming but really worth the effort. Companies that were not there fifteen or twenty years ago have indulged in this, and companies that have been their before that have been established as leading brands, more because of the product's utility and it quality. Names have become house-hold names, "thanks to branding."
The last word from me would be "Be Human in your touch with Employees" and if they still desert you, take it that they don't deserve good people like you. Let them feel guilty for leaving you while you don't have to feel sorry they left you.
This will go a long way in solving the problem on hand, and who knows the competitors will have to work overtime to retain their employees.