My naive mgo-based app file structure

database.go

import (
	"labix.org/v2/mgo"
	"log"
	"os"
	"regexp"
)

var (
	mongoSession *mgo.Session
	database     *mgo.Database
)

func connectWithDB() {
	var err error

	dbUri := os.Getenv("MONGODB_URI")
	if dbUri == "" {
		dbUri = "localhost"
	}

	dbName := regexp.MustCompile("([\\w-]+)$").FindString(dbUri)

	if dbName == "" {
		dbName = "my-db"
	}

	if mongoSession, err = mgo.Dial(dbUri); err != nil {
		panic(err)
	}
	log.Println("Connected to mongodb")

	database = mongoSession.DB(dbName)

}

func initCollections() {
	UsersCollection.Collection = database.C("users")
}

func init() {
	connectWithDB()
	initCollections()
}

user.go

package models

import (
	"instapray.com/crypto"
	"labix.org/v2/mgo/bson"
)

type User struct {
	Id bson.ObjectId `bson:"_id,omitempty"`
	Login          string `bson:"login"`
	Email string `bson:"email"`
	PasswordHash string `bson:"password_hash"`
	Firstname   string `bson:"firstname,omitempty"`
	Lastname    string `bson:"lastname,omitempty"`
}

user_handler.go

package main

import "mypackage/models"

func userHandler(w http.ResponseWriter, r *http.Request) {
  models.UsersCollection.DoSomething() // ...
}

users_collection.go

package models

import (
	"labix.org/v2/mgo"
	"labix.org/v2/mgo/bson"
	"strings"
)

type UsersCollectionType struct {
	*mgo.Collection
}

func (c UsersCollectionType) FindByLogin(login string) (User, error) {
	var user User
	login = strings.ToLower(login)
	if err := c.Collection.Find(M{"l": login}).One(&user); err != nil {
		return user, err
	} else {
		return user, nil
	}
}

var UsersCollection UsersCollectionType