The Cochrane Handbook has become the de facto standard for planning and carrying out a systematic review. Chapter 6, Searching for Studies, is most helpful in planning your reivew.
AHRQ Methods Guide for Effectiveness and Comparative Effectiveness Reviews
Very good chapters on conducting a review, most of which were published as articles in the Journal of Clincal Epidemiology. For searching see especially Finding Evidence for Comparing Medical Interventions and Selecting Observational Studies for Comparing Medical Interventions.
You will improve the quality of your review by adhering to the standards below. Using the approriate standard assures editors and reviewers that you have consciencously carried out your reveiw.
Institute of Medicine of the National Academies - Standards for Systematic Reviews
The IOM standards promote objective, transparent, and scientifically valid systematic reviews. They address the entire systematic review process, from locating, screening, and selecting studies for the review, to synthesizing the findings (including meta-analysis) and assessing the overall quality of the body of evidence, to producing the final review report.
Moher D, Liberati A, Tetzlaff J, Altman DG; PRISMA Group. Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. PLoS Med. 2009 Jul 21;6(7):e1000097. Epub 2009 Jul 21. PubMed
Liberati A, Altman DG, Tetzlaff J, Mulrow C, Gøtzsche PC, Ioannidis JP, Clarke M, Devereaux PJ, Kleijnen J, Moher D. The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate health care interventions: explanation and elaboration. PLoS Med. 2009 Jul 21;6(7):e1000100. Epub 2009 Jul 21. PubMed
Also published in the Annals of Internal Medicine, BMJ, and the Journal of Clinical Epidemiology.
Meta-analysis of Observational Studies in Epidemiology checklist contains specifications for reporting of meta-analyses of observational studies in epidemiology. Editors will expect you to follow and cite this checklist. It refers to the Newcastle-Ottawa Scale for assessing the quality of non-randomized studies, a method of rating each observational study in your meta-analysis.
Stroup DF, Berlin JA, Morton SC, Olkin I, Williamson GD, Rennie D, Moher D, Becker BJ, Sipe TA, Thacker SB. Meta-analysis of observational studies in epidemiology: a proposal for reporting. Meta-analysis Of Observational Studies in Epidemiology (MOOSE) group. JAMA. 2000 Apr 19;283(15):2008-12. PubMed PMID: 10789670.