It really depends on your general skill level and how deep you are into RoR. But I'll try to give a few generic tips.
Ruby and Rails are not just a programming language and a framework. They have an extensive set of rules and best practices that have been proven to work best in the long term.
So, first of all, read and learn by heart The Rails Way and Rails Antipatterns (this is a bit out of date - Rails 3 - but still an incredible source of things-to-avoid in RoR).
Learn to use rubocop and enable the Rails-specific cops. It will report style issues in your code, as well as methods that need some refactoring to make them easier to follow. And in most cases it can fix them automatically with the -a flag.
CodeClimate is another tool (free for open source) to check the quality of your code. It will run a suite of (mostly) open source tools to find style issues and other hotspots in your code.
This is not just for Ruby on Rails. You should test all of your code. If you still have not, learn the difference between unit and integration tests. Learn to use RSpec and Cucumber. And learn to make testing an integral part of your workflow. Get used to automated tools like Travis.
Rails has a huge ecosystem of gems, that has only recently been surpassed by NodeJS packages (which is not necessarily a good thing, given the state of JS). You should understand when it's time to use a gem, and when it may be better to reinvent the wheel.
When choosing a gem, learn to check it's health. Check out the number of GitHub stars and forks. Check whether it has an extensive test suite (some gems have a badge with the test coverage). Check whether it has large dependencies (do you really want to add 25 (mostly-unused) gems to your runtime memory?), and that they are compatible with yours. And most importantly check that it is up to date. If the last commit is from 3 years ago, run away.
This is a big issue with Ruby. Today we can assume that RoR can be scaled just like any other programming language.
In fact, it's not the language that dictates the scalability of a project, but how it's used.
Write thread safe code. Learn to optimise your DB queries and use appropriate indices (or if you wish, why not trying one of those new shiny NoSQL dbs?).
Learn to use cache correctly. Finding a good way to expire caches is one of the hardest thing in computer science (a quote from Phil Karlton).
Try to move to a Microservices oriented architecture.