Running an instance definitely does help, but the problem is convincing people your small instance is competently run and there to stay.

For people unable to selfhost, ways to help include:

• Making posts and comments, especially in smaller “dead” communities. Preferably about things other than Reddit.
• Contributing fixes on GitHub
• If you’re unable to do that last one, contributing quality reproducible bug reports on GitHub (rather than in, say, instance-specific meta communities)
• Helping to answer questions from all the confused new Lemmings, yes a lot of them are asked repeatedly but we want people to learn and stay so be polite and patient.
• Making sure people know about finding communities through sites like lemmyverse.net
• Curating lists of related communities and approaching mods to have it stickied or added to their community sidebar (helps people navigate around within their niche interests)
• Going to the extra effort of including local links when you mention a community, so people can find their way directly to the subscribe button. Example: !crochet@lemmy.ca (Lemmy / Kbin)
• And of course, shitposting memes.