Some common CRO techniques used by agencies include conducting A/B and multivariate testing, analysing website heatmaps and user recordings, optimising website forms and checkout processes, improving website load times, enhancing call-to-action (CTA) design and placement, utilising persuasive copywriting techniques, implementing personalisation strategies, and optimising landing pages for better conversions. These techniques help agencies identify and implement changes that can have a significant impact on conversion rates and business success.