You may know that the best way to build up your credit is to have credit. However if your scores are in the dumper you will find yourself with few credit cards willing to take you on and those that will be unabashedly predatory.
You can also get the information about ‘best cheaper credit’ (also known as 'am besten billiger Kredit' in the German language) and how it's helpful for getting a loan.
If you want to build up your credit and get something for your money you may want to think outside the box and build up credit lines with credit unions or retailers who will give you a better deal on your loan if you will buy their product. It can be a win-win for both of you.
The most economical way to quickly boost your credit profile is to open a secured loan with a credit union. Not all CUs offer this service but if you call around you can probably find one that does in an afternoon.
You simply give the Union a lump sum of cash to deposit in a savings account (usually interest bearing) and they will then give you an installment loan in the same amount secured to those deposits (which are not accessible until the loan is paid off).
Typically you would take 4-6 months to pay the loan back and then you can either take your cash out of savings or take a new loan.