What do big, high charging SEO agencies do to their big clients' websites to improve rankings when they have big budgets?

As they say, "content is king," especially in the arena of long-term whitehat SEO. There are no secret techniques that allow you to simply buy your way to the top of SERPs, and it's the job of good search engines to make certain of this by ranking results based on relevance and value to searchers. However, a large budget does afford you some advantages: While not SEO, SEM is a major part of online marketing and one where you can flex your budget to get results. Having a big budget also means you can spend more money to develop good content by hiring good copywriters, technical writers, brand ambassadors, bloggers, etc. Another major advantage is the tools you have access to. In addition to SEO-specific tools like keyword research, rank tracking, and competitive research tools, enterprise products often integrate multiple marketing channels (including inbound/outbound marketing), marketing & sales automation platforms, tag management, enterprise social media management, etc., and provide richer analytics, reporting and BI tools. Successful organizations understand that marketing is a continuous and iterative process, which SEO and inbound marketing is a part of. So another major part of the budget is continuous testing, reassessment, and refinement of strategy, which is often the job of an inbound marketing consultant. All of this taken in together makes the large budgets allotted to online marketing much easier to understand, and (especially the last component) helps to ensure accountability and a high ROI on marketing spending. But generally large companies don't treat SEO as a siloed activity on its own. Even if they hire a specialist SEO consultant, they're usually part of a larger marketing team that coordinates all marketing activities, including branding, creative, direct marketing, SMM, etc.

We can do a lot of things when we have a huge budget. You can create unique content, campaigns, sem, offline promotions, contests, social media events, ads etc. Remember, an online promotions never limits with seo. You taste success only when you treat your website promotion as a whole & single entity. Think about only how can you bring traffic to your website....