I use direnv to automatically get into flake shell as soon as I entre the directory which contain the flake.nix, the content of .envrc file is:

use flake

Most of the time I just need to use the package available in my shell for my dev work and the snipet that I use is:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };
  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem
      (system:
        let
          pkgs = import nixpkgs {
            inherit system;
          };
          buildInputs = with pkgs;
          [
          # package1
          # package2
          ];
        in
        with pkgs;
        {
          devShells.default = mkShell {
            inherit buildInputs;
          };
        }
      );
}

Sometime I write custom scrips that I need access to, I can create a nixified version using the writeShellApplication which is one of the trivial builders. Bottom is just a simple shell script to get the authentication code simlar to google-authenticator:

{ config, pkgs, ... }:
with pkgs;
writeShellApplication {
  name = "totp";
  runtimeInputs = with pkgs; [ cloak ];
  text = ''
    cloak view some_app | tr -d '\n' | pbcopy
  '';
}

and for new projects I use nix flake templates from nix flake show templates using nix flake init -t 'templates#go-hello'

I will keep adding to this post if I find someting that I use often.