Refusal of visit visa

I applied for visit visa from Pakistan in mid of 2010 along with an invitation latter, but its rejected with reason they doubt I will not return back and don't have strong ties in Canada, I again applied visit visa in 2011 along with my company financial reports, they again reject with reason they are not satisfied with the purpose of visit, I have few question
1,   can I apply again? Do they consider blacklist if a applicant rejected twice or thrice?
2,   can anyone give me tip for visa submission?
3,   I also applied in QIIP in September 2010, my visit visa refusal will affect my Immigration file?
Shakir Ali Khan
shakir @ frcpk.com