Doing signature campaigns is pretty safe to grow your bitcoins and with a good campaign it can be very profitable, what of course is important for us all.
Trading is also a option but the risks are pretty high and you can lose money from it real fast.
I agree that signature campaign is the cheapest and the safest way to grow your bitcoin but the recent demand in Signature Campaign negates member below rank and in most cases only from Sr, Member and up were accepted. But at least there is signature campaign that pays in altcoin and letting even jr. member rank to join the campaign.
Trading altcoin really is very risky especially when you invested in some PnD coins.
The safest way to earn and grow your bitcoins is through signature campaign where you get fixed amount of coins on regular basis and if you can manage to save those earnings then over a period of time you will be holding good amount of coins in your wallet and if you can manage to take certain level of risk then invest those earning in trading to grow them quickly.