How to migrate work items from TFS to Visual Studio Team Services

My team currently works with an on-premises TFS 2012 server. I am migrating everything to Visual Studio Team Services, formerly Visual Studio Online. I am starting with a test project and was able to easily get all the code migrated, but can't figure out how to do the same for the work items.

Are there any good guides out there?

New options as of March 29th 2018:

TFS to VSTS migration - The official import option which will import 1 project collection into 1 VSTS account. It automatically imports everything stored in the backup. At the point of writing this, the TFS must be upgraded to TFS 2018 and some work item template customizations must be removed (there are a few well documented features unavailable on VSTS).
VSTS Sync Migrator - Marting Hinshelwood, the uncrowned king of TFS and VSTS migrations, has built his own little tool that can migrate work items from one server/account to another. It can even do migrations from one Team Project to another and while doing it switch between process templates.
VSTS Work Item Migrator - Microsoft has also open sourced a project that they used internally to migrate work items. It's less powerful, but it was made by Microsoft.

Previous answer:

At the moment there isn't a really good story. Your options are:

Start over - easiest :).
Start over and manually recreate items of value - It's a pain, but it's some teams have done these things in the past. keep the old TFS server available in read-only mode and each time you use a work item in the old system, you manually create it in the new one, set all the fields and upload the attachments. Depending on the number of items it'll take you a few sprints to migrate the most important stuff over.
Wait a while longer - Microsoft is currently working on a full fidelity import option which will allow you to upload a Project Collection and it will be exposed as a new VSTS Account (it's not going to be possible to import a project collection into an existing account).
Use Excel for import/export - Will work for most work items, you loose attachments and work item links other than parent/child. The trick is to extract from one Project Collection then copy all fields, except the ID to an Excel sheet bound to the target project collection. You will need to fix all Identity fields (works best when users have the exact same display name on premise as in VSTS) and you'll have to import once with state new and then past the current state/reason over the just imported values and sync again. Test Cases, Plans, Suites and Shared Steps will not be imported with their relations in tact. The approach would be very similar to this one.
Use the TFS Integration Tools - Will work for most work item types, though it will loose custom kanban states and tags. Test cases, Shared steps and their relations will not be imported. This option will allow you to import import work items and source code with their relationships in tact.
Use a 3rd party solution - Out of the available options currently OpsHub offers the most complete solution. For test case and source control link migration you're looking at the commercial edition, which comes at a steep price. It still has a long list of known issues and last time I tried it, I ran into numerous issues which required their support to resolve them.

There are specialized TFS consultants who live off these kinds of migrations if your current state of the work items is precious to you, then you could reach out to them.